Simple Interface

A number of useful drawing tools are made available. (See Drawing Tools, below.)
The drawing canvas is a fixed size, so the user doesn't need to worry about 'pixels' or 'inches' when making a new picture.
The entire program fits on a screen as small as 640x480, and can be displayed full-screen, to hide the computer's underlying 'desktop' interface. (It defaults to 800x600 mode, but can run at larger sizes and in portrait or landscape mode, as well.)
Loading and saving of images is done using thumbnails, so no knowledge of the underlying operating system's filesystem structure is needed. Even filenames are unnecessary.
Entertaining Interface

Fun sound effects are played when tools are selected and used.
A cartoon version of Tux, the Linux penguin, appears at the bottom to give tips, hints and information.
Drawing Tools

Paint brush
Various brushes available
Parents/teachers can add more brushes
Brushes can be animated
Brushes can change shape depending on the direction they're drawn
Alternate color palettes can be created and used
Rubber stamp
Hundreds of photographic and cartoon stamps images are available
Parents/teachers can add more stamps and create categories
Stamps can have descriptive text and sound samples (names, facts, etc.) and/or sound effects assigned to them
Stamps can be resized, flipped and mirrored
Raster bitmaps (Portable Network Graphics ("PNG")) with full-color and alpha transparency (32bpp RGBA) supported
Vector graphics (Scalable Vector Graphics ("SVG")) supported
Line tool
Uses Paint Brush's brushes
'Rubberband' shows where the line will be drawn as you move the mouse
Shape tool
Draw various filled and unfilled polygonal shapes
Rotate shapes (this feature can be disabled for younger children)
Text and Label tools
Come with various fonts
Bold, italic and size can be changed
Parents/teachers can add more fonts
Supports numerous languages (See International Character Input, below)
Labels may be moved or edited later (Coming soon)
'Magic' (special effects) tool
Blind - pull window blinds down, or across, the picture (Coming soon)
Blocks - turn parts of the picture 'blocky'
Blur - blur parts of the picture using the mouse
Bricks - paint realistic large or small brick patterns
Calligraphy - a smooth brush that changes thickness depending on how quickly you stroke
Cartoon - make parts of the picture look like a cartoon
Chalk - make parts of the picture look like a chalk drawing
Color & White - turn the picture monotone
Color Shift - Jumble the colors of the picture
Confetti - Throw confetti over your picture
Darken - darken the colors of parts of the picture
Distortion - slightly alters the position of the picture under the brush
Drip - make parts of the picture look like it's dripping away
Edges - outline the edges of shapes in yur picture
Emboss - gives the image's edges highlights and shadows
Fill - 'flood-fill' an area with a particular color
Fisheye - view parts of your picture through a fish-eye lense
Flip - flip the image vertically
Flower - draws curvy-stalked flowers
Foam - paints foamy bubbles that grow as you move the mouse
Fold - fold down the corners of your picture
Glass Tile - see your drawing through glass tiles
Grass - paint realistic clumps of grass on the screen and into the distance
Hexagon Mosaic - create a pattern of hexagons (Coming soon)
Irregular Mosaic - turn your picture into a mosaic of shards (Coming soon)
Kaleidoscope - four symmetric brushes
Light - draw beams of bright light on your picture
Lighten - fade the colors of parts of the picture
Metal Paint - paints shiny metallic colors
Mirror - flip the image horizontally
Mosaic - make your picture into a mosaic of colors
Negative - invert the colors of parts of the picture
Noise - add random noise to your picture
Perspective - bend your picture to view it from different perspectives (Coming soon)
Picasso - a triplet of brushes that paint in unison
Puzzle - shift parts of your picture around like it's a sliding puzzle (Coming soon)
ROYGBIV Rainbow - draw a rainbow arc of seven solid colors (Coming soon)
Rails - draw connecting train tracks
Rain - add raindrops to your picture
Rainbow - draw using a brush that paints in all colors in the rainbow
Real Rainbow - draw a colorful arc based on a real photo of a rainbow
Ripples - rippled waves over your image
Rosette - three brushes that draw in a rosette pattern
Shift - move the entire picture around
Silhouette - turn parts of your picture into a silhouette
Smudge - push the colors around as though smudging wet paint
Snow Ball - cover your picture in snowballs
Snow Flake - cover your picture in snowflakes
Square Mosaic - create a pattern of squares (Coming soon)
String V - draw V-shaped string art
String corner - draw string art
String edges - draw string art around the edges of your picture
TV - make your picture look like it's on a television
Tint - change the color of parts of the picture
Toothpaste - paint with colorful gel
Tornado - add a tornado funnel to your picture
Wavelets - wiggle your picture up and down
Waves - wiggle your picture side to side
Wet Paint - draw with wet, smudgy paint
Zoom - zoom your entire picture in or out (Coming soon)
... plus, programmers can easily and quickly create new 'Magic' tools using a plug-in interface!
Eraser
Undo
Multiple levels of undo
Undo actions can themselves be undone with the 'Redo' button
Commands

Save
Save a picture to the virtual 'picturebook'
One-click save: No file browser dialogs or filename prompts are used
If saving an opened picture, choose to save over it (like 'Save' in desktop programs), or save a new picture (like 'Save As')
Open
Open a previous drawing by selecting its thumbnail in a virtual 'picturebook'
Delete previously-saved pictures
Slides
View a slideshow of multiple saved drawings
Control the speed of playback; create class presentations or simple looping animations
Go forward or back using the arrow keys or mouse
Starters
Various coloring-book-style images included
Photographic scenes included, with foreground and background elements that you draw in between
Vector graphics (Scalable Vector Graphics ("SVG")) supported
Kid Pix templates ("KPX" files) supported (Coming soon)
Print
Print pictures on a printer
Translations

Parts of Tux Paint have currently been translated into the following languages:

Acholi
Afrikaans
Akan
Albanian
Amharic
Arabic
Aragones
Asturian
Azerbaijani
Basque
Belarusian
Breton
Bulgarian
Catalan
Chinese (Simplified)
Chinese (Traditional)
Croatian
Czech
Danish
Dutch
English (Australian)
English (British)
English (Canadian)
English (South African)
English (USA) (default)
Esperanto
Estonian
Finnish
Faroese
French
Fula
Irish
Inuktitut
Gaelic (Scottish)
Galician
Georgian
German
Greek
Gronings
Gujarati
Hebrew
Hindi
Hungarian
Icelandic
Indonesian
Italian
Japanese
Khmer
Kiga
Kinyarwanda
Kinyarwanda
Klingon (Romanized)
Korean
Kurdish
Latvian
Lithuanian
Luganda
Luxembourgish
Macedonian
Malay
Malayalam
Marathi
Ndebele
Northern Sotho
Norwegian Bokmċl
Norwegian Nynorsk
Occitan
Ojibway
Persian
Polish
Portuguese (Brazilian)
Portuguese (Portuguese)
Romanian
Russian
Serbian (cyrillic)
Serbian (latin)
Shuswap
Slovak
Slovenian
Songhay
Spanish (Mexican)
Spanish (Spanish)
Swahili
Swedish
Tagalog
Tamil
Telugu
Thai
Tibetan
Turkish
Twi
Ukrainian
Venda
Venetian
Vietnamese
Walloon
Welsh
Wolof
Xhosa
Miahuatlán Zapotec
(You can even help translate it to your favorite language.)

International Character Input

Enter text in different languages using the Text tool:

Latin languages - Spanish, French, Italian, Portuguese, etc.
Japanese - Romanized Hiragana and Romanized Katakana
Korean - Hangul Bul-2
Thai
Traditional Chinese
Parental and Teacher Controls

Run Tux Paint in full-screen mode, or have it 'grab' the mouse pointer, to prevent children from accidentally accessing other programs or files on the desktop.
Sound effects can be disabled.
The Print and Quit options can be disabled.
Alternatively, the Print option can be limited to allow only one print every n minutes.
Tux Paint cannot be launched more than once every 30 seconds, to prevent multiple copies from being run accidentally, due to over-eager mouse clicking.
The directory/folder where Tux Paint saves files may be changed. This can be used to save pictures in a student's home directory on a network drive or shared drive, rather than locally on one particular workstation.
Stamps, starters and brushes are stored using popular open formats (PNG, SVG, Ogg Vorbis, etc.) allowing parents and teachers to create their own content for use at home or in the classroom - even using completely free tools.
The 'Tux Paint Config.' program allows parents, teachers, and school technicians to alter Tux Paint's behaviour using a simple, easy-to-use graphical interface. (It's separate from Tux Paint, to prevent children from changing things.)
... and much more!