A textarea with code completion and syntax coloring and a complete control over the caret position.
here a demo

For example: write window. and press ctrl+space... :)
ctrl+l --> goto line

todo:

• Support for large documents
• Undo, redo
• Copy, cut and paste
• A better colorization algorithm
• Line numbers
• Code folding
• A lot of stuff!

nic
ps now it works properly only in Firefox and IE