http://www.twilert.com/
be alerted when someone tweets about something (even if it doesn't make it into the pages Google pulls info for its alerts)

http://www.socialmention.com/
be alerted when someone mentions something on social sites, such as stumbleupon, digg (even if it doesn't make it into the pages Google pulls info for its alerts)

And, of course, don't forget to set up Google alerts