I was writing going to write up an article for my site tonight to show how to create a table of contents using native Excel functions, rather than resorting to VBA. Naturally, I figured that it would be great to put up an interactive example with the Excel WebApp, but I ran into an issue: I found a formula that works fine in the client, but just returns #VALUE! Errors in the Excel WebApp.
Debra did a great writeup of the =CELL function back in her 30 Excel Functions in 30 Days series. The examples work great in the client, but not the webapp. Too bad, really, as it's a great function that can be used for a lot of things.
I don't know how much people have played with this, but if you encounter a function that doesn't work, post it in the comments. It would be nice to get a full list.