Trackur has one of the largest list of sources in the industry--too many to list. Some highlights:

All mainstream news sites 
Blogs 
Google+ (paid plans only)
Videos (including YouTube)
Images (including Flickr)
Forums and Message Boards (paid plans only)
Twitter 
Facebook (paid plans only)
Reddit 
Delicious

With our Ultimate plan, you can also upload the RSS feed of any web site not currently being tracked by us.