Map of the London underground and various bus maps for mobile phones. Runs on any recent Java enabled phone. London underground also has an official map midlet but it is less good than an empty freight train.
This is a mobile application that lets you view offline (i.e. no data charges!) maps of the UK. The most useful currently available one is of most of London. Runs on many Java enabled phones, but some are buggy and don't support it. Advantages compared to other solutions:
Runs on cheap 'feature phones'
Map data stored offline so it is both fast and free.
Searching uses a predictive-text style input and is instant.
It uses streetmap.co.uk's maps which fit more on a single screen than other maps.
C++ wrapper around libsmbclient. Actually some parts are still left in a C-like state but it is still kind of useful. I used this and libsqlmm to create an SMB search engine that was awesome at university. Some people rewrote it in PHP after I left because they weren't clever enough to handle it's awesomeness.