Web Application: IDE
I'm not sure I really need a simplistic IDE as a Web application. Then again, it might be handy to try some code from my phone or at someone else's PC. If you think that sounds right, have a look at IDEONE.
Calling it an IDE might be a bit of a stretch since it looks to me like it only handles one file at a time. If you click "syntax highlight" at the bottom right it does look a little more like an editor, I guess.
The really interesting part, though, is it handles 40 different languages ranging from Ada to Whitespace. Some languages that it supports, like Forth, suffer from being compiled and run on the click of a submit button, but that's life in a Web application, I suppose.
You can also get sample code and templates for each language and see what other people have been running in their IDE recently (although you can mark your code as private).
So tonight when I have an urge to try to remember exactly how a FORTRAN FORMAT statement works, I can log in to IDEONE and get it working. Whew!