Twitter kicks ass. I'm sure it's just as terrible as most young people seem to think if all you're doing is following your high school friends and reading about what they had for lunch, but it's an amazing service if you use it right.
My favorite use is simply posting jokes. FATJEW is my favorite twitter user ? his twitter fed is one of the funniest things I've ever read on the Internet.
Beyond that, it's great for following news. This can be official news ? I get an update every time the Black Mesa mod team makes a major update, for instance ? or it can be live-tweets of events, such as when a couple of my friends live-tweeted a disqualification appeal hearing at my university's student society.
There's all kinds of other bizarre features as well. Many twitter apps allow you to tag your longitude and latitude on your tweets, so other users can see where you were when you posted. This is useful for all kinds of things. While the Olympics were still on, for instance, a friend of mine wanted to know if the lineup for Germany House was too long to be worth it; he pulled out twitter and scanned for tweets coming from that area in the last few minutes, and saw a bunch of complaints about the four-hour lineup. Thanks to Twitter, he didn't have to waste his time going down there to check for himself, and was able to go to a different venue that wasn't so busy.