Historical version 1 of Metro
Metro is a free application available for Palm and for Pocket PC handheld computers, for finding routes on public transport. London is, of course, covered, along with hundreds of other cities. The London data files include details on many places of interest. Due to memory constraints, bus routes aren't included (London has far too many!) but there is a seperate database for the Night Bus network.
On Palms, the application plays nicely with the Address Book, allowing you to specify your contacts' nearest stations and find routes to people as well as just to stations.