Google Book Search has added a few magazines to their repertoire.
Today, we're announcing an initiative to help bring more magazine archives and current magazines online, partnering with publishers to begin digitizing millions of articles from titles as diverse as New York Magazine, Popular Mechanics, and Ebony.
At least I think it's a few magazines...it might be thousands but there's no way (that I can find) to view a list of magazines on offer.
Update: Spellbound and Thomas Gruber have lists of some of the magazines on offer.