Last weekend, I went to Mashed 08, run by my employer, the BBC
During the first morning, several APIs were released and a competition was started with 24 hours to hack/mash an application together, using one of those APIs.
As a BBC employees, myself and my colleagues decided to create an application related to our roles, which it turned out what a "personal, pocket, travel guide" application, which allowed you to create your personalised travel guide for any given city and choose which points of interest you wanted in your guide (we created PDFs).
The APIs we used to create this were the Lonely Planet Developer Network and the Multimap Open API. These allowed us to gather destination and point of interest information (including geo information) and then plot these on the map (that every good application should contain!)
Overall, the weekend went very well, I personally had great fun developing again, and the two APIs we used were very complete, very easy to use and well documented (for what we needed).
We had many enhancements we could have done to the application, but had to succumb to sleep at some point!
Thanks go to the BBC for running the event, apparently last year was just as much fun, but I missed it.