It would be nice to see a built-in spell checker into the main text-panel for posts. Every time I post here or Stack Overflow I quickly open up Gmail, paste my post there, and use its spell checker to make sure I didn't make any stupid mistakes. I know other services have these types of features, and it would be nice to see Stack Overflow have one as well.
(BTW, I found two small mistakes for this short post.)
I know I made a comment about this on ChrisF's answer, but I thought that I should also mention it here in the question. I know there are many third-party tools to add spell checker functionality to Stack Overflow. But some (if not many) people out there do not have these tools. They are either at work (like me), or some other place where they using a machine that is not owned by them, or they aren't power users and just don't have these tools installed.
If Stack Exchange becomes used for other non-technical sites, then you will probably have to count on the fact that people are using Internet Explorer 6 or something just as bad. That's why it would just be nice to have a built-in spelling tool. Again, it may not be worth it, but I think it would be interesting to discuss it at least.