There are a few professional paid apps available for designers and digital illustrators. But there are also other options available, like these free online tools listed below, for drawing, sketching, painting, photo editing, etc. These tools may be not as professional, but it might come in handy next time..
(side note: if you found another resources of online tools, you can give comment containing the links so it can be added here)
DRAW, SKETCH, PAINT
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Draw.to (super simple)
Draw something and share it!℠
Sketch swap (super simple)
Draw something and trade it with a stranger
You draw something on the screen, and when you’re finished, you hit “Submit drawing” to receive a random drawing from someone else. (needs Flash 8+ and JavaScript enabled)
Wiiplayable (simple)
draw with markers using your wiimote
InspirARTion (pretty complex)
Sketch & Draw
Try symmetry modes to create interesting effects.
Kerpoof (pretty complex)
Crayola digi-color (pretty complex)
http://www2.crayola.com/coloring_application/index.cfm?referrer=/index.cfm
Draw Island (pretty complex)
an online drawing tool where you can create nice draws and animate images too.
DeviantART muro (complex)
a browser based html 5 drawing app
It has a bunch of features such as 20 brushes (some free, others require payment), layers, and different filters. You can use the dA Muro with a WACOM tablet.
(Note that in order to use Muro, you must be a member of deviantArt but joining is free.)
Slimber (complex)
Drawing and Painting Online
It allows users to draw online, replay and save their drawings. Users can rate, comment and share drawings. The Most Rated drawings are rewarded by being featured on the home page.
Fat Paint (complex)
Graphic Design Software, Logo Maker, Photo Editor, Page Creator, Paint and Draw Program. Create Pictures and Illustrations.
Wixie (complex)
SketchBookCopicEdition (complex)
download it first
http://copic.jp/en/sketchbook-ce.html
https://itunes.apple.com/en/app/sketchbook-copic-edition/id447678864?mt=12
FANCY STUFF
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Sketch Toy
An Online 3D Sketch Tool that uses HTML Canvas to create 3D drawing.
To rotate the canvas in 3D, hold down SPACE and then DRAG horizontally with your cursor.
Sketchy Structures
velocity sketch tool
An HTML5 canvas drawing tool
Scribbler
a generative illustration toy by ZEFRANK.COM.
The Scribbler takes simple vector based input (your drawing) and creates its own drawing on top of it based on a number of simple rules.
Pixlr.com
Online Photo Editor, you can draw too!
FOR WEB DESIGNERS
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Sprite Box
Spritebox is a tool to help web designers quickly and easily create CSS classes and IDs from a single sprite image.
It is based on the principle of using the bacground position property to align areas of a sprite image into block elements of a web page.
Font Dragr
Drag and drop font testing
Test custom fonts in the browser. No coding, no uploading, just drag and drop.
All you need to do is drag and drop a font file from your computer into font dragr in a browser.
You also can test on any website. The bookmarklet allows you to test any font from your file system or any of the fonts found in the gallery, on any website.
XRay
A bookmarklet that allows you to quickly view details of any element on any webpage.
It also can handle HTML5 elements such as canvas and provide you with the design information you need.
SVG to HTML5 Canvas Converter
This tool converts SVG (Scalable Vector Graphics) into an HTML5 Canvas JavaScript function.
→ 10 Super Useful Tools for Better Web Typography
http://webdesignledger.com/tools/10-super-useful-tools-for-better-web-typography
DIAGRAMMING
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Draw.io
A free online diagram drawing application for workflow, BPM, org charts, UML, ER, network diagrams.