Twitter's great. It's like a secret club where people in the tech sector go to talk about the inanities of their lives... and it's horrendously addictive. It may surprise you to learn though that Twitter can also be used for some genuinely useful things. ... Okay, that was obviously a lie. But there are many things you can do that will enhance your Twitter feed, and give it the virtual 140-character equivalent of bells and whistles. For this week's mash-up, I'm going to go through some services that you can hook into Twitter.