The Twitter API allows identification of the client used to tweet when sending them. This basically puts a link at the end of each tweet to the client website, allowing everyone to see what the app is and therefore publicity for same.
I recently put in a request for Twitux and they approved it. The guys at Twitter sent me an e-mail as follows:
Thanks for requesting a source parameter link for your application,
Please have your application send a parameter named "source"
with a value of "twitux" when you POST updates to the Twitter API.
The link to your application should show up on the Twitter site
in the next 24 to 48 hours.
Thanks much, and good luck with your application!
Hope this makes it's way in to the code :)