Every so often, a big site will make a change to its terms of service that's not so great for its users—without even informing them—and they (understandably) get really pissed off. Recent examples are Facebook's and Instagram's changes, which resulted in user backlash.
They'll probably never be proactive about letting us know what's going on, so that's why Docracy created their Terms of Service Tracker. It tracks the changes and keeps you up to date on the TOS of "hundreds of the world's top sites" like Twitter, Instagram, eBay, and GitHub (and more are added all of the time).
Anything taken out of the TOS is marked in red, and the additions are green.
Clicking on a site will give you a short explanation of the changes and display them, if there aren't too many.
The majority of the changes are really simple and innocuous, like going from http to https, but it's nice to know that if a site you use frequently ever does make major changes to its agreement, you can easily find out. If they're going to do something that affects your privacy or personal information, you have a right to know.
To see a full list of the sites and detailed info on recent changes, check out the full directory.
If you notice the terms have changed on one of your favorite sites, but like most Internet users you're too lazy to actually read the damn thing, check out Terms of Service; Didn't Read, which spells them out for you in plain English.