TubeBuddy is a free application that gives you access to critical information about the London Underground (Tube) from your BlackBerry. It provides live DLR service updates along with live departure schedules. They recently added OS 5.0 compatibility along with direct APN internet connectivity for people roaming in the UK.
TubeBuddy is a nice little app that I wish I had for the NYC subway. It provides you with up to the minute departure boards telling you when the next train is scheduled to leave. You can rearrange the lines in the application so you do not have to scroll through them all every time. Problems on the lines will be prioritized so you notice them.
The only downside seems to be that there is no built in overall map of the Underground system. I know in NYC this is because the MTA charges a license for the map so it could be the same in the UK. Still for a free app it looks pretty useful.