Tesco groceries. I LOVE that I can just scan items to add them to my basket for delivery. Also, if you have to take the LIUK test, there's a great app for studying that as well.
I agree with the tube app. It's wonderful!
Another good one is Skype, as you can call international from your mobile from your skype credits.
Edited to add: If you are still in the US, it's a good idea to download Textfree ( Don't think you can get the US version here but I could be wrong) I can text my kids in the states with it. I can't text international from my mobile without it.