If you've posted recently, then you've noticed a new "Check spelling" link above the upper right hand corner. The "Check spelling" link is enabled both for regular posting and for the quick reply.
Using it is absolutely seamless and easy. Just click the link, and it checks your text for spelling errors right then and there. No pop-up boxes, no annoying ads from spellingcow, and almost no extra time necessary. Of course, if you're using Firefox, then all of this is redundant for you.
What's more, though, is you can change the default language, which is useful in .001% of all posts. Still, a cool feature for those planning to write in Polish or Spanish.
The program is all done with Googiespell, available here:
http://www.orangoo.com/labs/GoogieSpell/
So cool program, cool feature. If you want to thank someone, go to the above link and thank the author because I had to do absolutely nothing to fit it for YWS.
