Movies - Latest movies, boxoffice info etc
Facebook - Great fb application with alerts.
Viigo - RSS application that comes with a whole bunch of feeds set up and you can add/remove/edit your own.
Ubertwitter - Good twitter application, very quick.
Tube Map - London underground map application with live updates.
Opera 5 - Better browser then the standard blackberry one. You'll need to access the download link using your BIS as it wont let you download it over wifi for some reason.
Google Mobile, Sync & Maps - Maps is better than the blackberry one it comes with. Sync is useful for backing up your contacts, calendars etc
Nimbuzz - All in one instant messaging application with alerts.
Poynt - A bit like places directory, useful for finding local restaurants, shops, cinemas.
Shazam - Music identifier/tagger.
Time - Time magazine's own mobile application.
iTookThisOnMyPhone - Photo uploader.
Also you may want to get the latest version of
blackberry messenger.
There's a sat nav application called Telenav that you might want to try, not sure if it needs an additional bluetooth gps unit or if it functions with the built-in one.