TOSBack.org is a project run by the Electronic Frontier Foundation that tracks changes in the Terms of Service of various technology companies, from cell phone companies to social networking sites and beyond.
The really cool thing is every time a TOS changes, TOSBack.org displays the old and new TOS side-by-side with all changes highlighted so it is easy to tell exactly what has changed (although figuring out what all that legalese actually means may not be quite so easy).