How can i organize a contest in twitter?

[Alvaro Fernandez]

Let's say I want to give away a T-Shirt or a dinner for 2 at my website. I want to give it to someone in twitter who has posted a message including a specific #hashtag, that way encouraging many users to send a message (therefore promoting my site). I have seen many people in twitter doing it, so I know it can be done, any idea of how? any software suggestions? Free is better, but low cost alternatives could be considered.
I found http://tweetaways.com/ but then i read an article complaining about them having too many bugs, so if there are alternatives, i would like to get to know other options

[Cyan76]

Try http://tweetaways.com/

They let you specify a twitter phrase to look for and then you can choose a random winner - for free.