If your on a Mac, it’s built into safari. Just control click the text area window and tell it to check as you type.
I’ve found most web based checkers to be obnoxiously slow. If I can copy the text into a document, spell check it, paste it back into the web page faster than the online spellchecker, then well, the online one isn’t worth anything.
Looks like there are some great alternatives for the Windows folks too.