A few days back I was checking if there were spell check plug-ins available for WordPress. In the search I found an interesting article which told me about how the Google toolbar has a spell-check feature which works pretty well in the “Write Post” textbox too. So I tried it and it seems fine. For somebody who hasn’t tried it, here is quickie on it:
- You just type your post in the box.
- Click on Check, do the correction as maybe suggested.
- IMPORTANT: Before you hit the SUBMIT button, make sure you do a “Stop” on the spell-check or there is a chance to have some gibberish pasted on your blog.
The spell-check works fine with just about any other text box you may type into online. Like maybe a comment box. Neat, isn’t it?
Kapil's Korner 
Or you could always use Firefox which spell checks long forms just like Word does, putting the red line underneath the misspelled words and you can also right click and spell check the box.