Why stop at an IM client?
I'd like to see spell checking built in to all text boxes in Windows, right there on the context menu with 'Cut', 'Copy' and 'Paste'. I'm surprised we don't have this feature already.
I'd also like it if I could get IE to write a log file with all post data before it sends it to the server so that:
1) It's easy for me to see what is actually being sent
2) If the server fails I don't lose my work
3) I have a record of the data that I have given to any given web site.