I recommend the 3G Watchdog, interesting to know how much date you're actually using each month. You can also put in your monthly allowance, i.e 500Mb or 1Gb and the date of each month your contract is paid, and you can have a little icon on your homescreen telling you your % used to date and what it predicts you will use by the end of the month.
In addition to the other ones listed above, Barcode Scanner is a nifty little app, along the lines of Google Goggles but especially for barcodes. Bluetooth File Transfer also comes in handy and makes moving files by B.T. nice and easy.
TrainTimes Uk also covers all your train time requirements. Can use it as a widget on one of your home screens to show the times of the next few trains leaving your chosen station and if they're running late etc.
I can't remember how I did it, but I downloaded a programme off the tinterweb which able voice activation on my phone. For instance you can just hold down the search key for a second, say something like "Navigate London Zoo" and bingo it automatically goes into the Navigation programme and prepares your route to the Zoo. Can also dictate text messages etc. Top stuff.