Stekpad!

TiddlyWiki is a single html file with all the characteristics of a wiki - including all of the functionality (including editing, saving, tagging and searching) and the style sheet. Because its a single file, its very portable - you can email it, put it on a web server or share it via a USB stick. It has a broad range of tools and an ever expanding plethora of plugins.

Etherpad is a highly customizable open source online editor providing collaborative editing in real-time.