The “Kahuna” or Live Mail team announced a sneak preview of what’s to come. Spell Check as you type. As Imran Quareshi pointed out, no other free-webmail service (or paid webmail service as far as I know) has this feature built into it. This is something that no matter how minor or trivial it may seem, pretty much everyone has muttered at some time or another “I wish they had spell-check.”
You can find more information about the Spell-check feature, along with other information about Kahuna and other Windows Live projects at the links above. Also, you should check out http://www.live.com (a homepage for your web browser, that you can customize) and http://ideas.live.com (where they list all of the live.com features). On the live.com personalized homepage, if you sign in with a Microsoft Passport, the features that you customize will follow you to other computers (including any favorites that you upload). This is the culmination of the start.com sandbox page, and the Roaming Favorites beta program.
Have a great day, and a Happy Thanksgiving.