As this API is just entering public beta, it is highly advisable that you let me know if you build an application so I can contact you should any of the API details change - it's also great to see what people are building! Alternatively, you can subscribe to the blog on my website or follow me on Twitter for all of the latest updates.
I'm a PHP Developer and a while ago I created an XML feed of the current status of the tube lines according to the TFL website by screen scraping their site. I did this in order to power a twitter-based application I was writing at the time. I wrote about it on my blog in order to pass on knowledge of how to scrape pages with PHP but seem to have picked up a huge number of followers who use the provided XML feed for their own applications. It wasn't until I'd received several emails and an Apache access log that was building up at the rate of over 1GB a week that I decided to rethink the XML feed and provide a much better service.
Thus, tubeupdates.com was born. My idea was to have a fully featured REST-ful API that would allow any developer to instantly query what state the London Tube network was in. As a bonus, this would include the Docklands Light Railway and if possible would feature the individual messages that each line has in order to specify exactly why there is a delay or if any stations are closed, etc. A distant dream was to be able to search back through the various status changes over time so that other developers could build applications such as reliability graphs or time checkers.
I'm happy to say that all of the above is now available to any developer via a simple REST-ful interface. In addition, you can request either JSON or XML responses and it's all performed through one simple method!
To start using the API, take a look at the documentation.