Online code editing
I always keep an eye on what is happening at the Mozilla Labs and recently they launched Bespin which is an open source web-based framework for code editing.
Basically its an online IDE which supports for basic editing features, such as syntax highlighting, large file sizes, undo/redo, previewing files in the browser, importing/exporting projects.
Another alternative online IDE that I know of is Amy Editor which I think is best used if you specify you own server.
Another one maybe a little bit different is codepad which is an online compiler/interpreter and I think its best maybe for testing code snippets for the languages listed which include PHP, Ruby, Python and C++.
I think Mozilla Labs are on the right path here and I will be following the development of Bespin over the next few months where it leads to.