Open API from Twitter is a good base for many mashups uses Twitter's data. Here is another idea - information server on the top of Twitter. Twitter 411 service lets you easily build your own information system, uses Twitter as a transport. The idea is very transparent. You can define your own keywords as well as data, associated with the selected keywords. Users will ask service via direct messages in Twitter and receive responses as direct messages too. Direct messages for the requests are just user-defined keywords. Direct messages for the responses are just user-defined reactions (data associated with the keywords).
User defined responses could be provided as some texts (of course, the keyword owner will be able to change/update the text any time) or (what is more interesting) as URL's for the external CGI scripts. So you can reserve a keyword and provide your own CGI script for the responses. The engine simply calls this URL in order to get the response. And the text of the original request could be passed as HTTP parameter.
The usage for this service is transparent. Just follow to t411 in your Twitter and send direct messages to t411 with your requests.
Some examples or build-in services:
test - just a test for the service
t stock_ticket. E.g.: t JAVA - returns price for the ticket. This service uses an external CGI script for getting quotes.
And by the way, it is automatically a mobile service too.
Technically, mashup uses appropriate components (including Twitter taglib) from Coldtags suite.