If you're looking for five minutes of fun while you wait then Cube Runner is a great free game. You navigate a little ship, little more than an arrow really, through mine fields of cubes of increasing complexity and at ever higher speeds. It's not too graphically intensive, so won't drain your battery extensively, while the tilt controls are nicely tuned and work very well indeed.
Tetris clones are nothing new but there's a good reason they exist: Tetris rocks. Tris is a solid and good looking example. It takes a little while to get used to the touch controls, you need to be pretty accurate when dragging blocks down, but once you've got used to things it's just as fun as Tetris has ever been. Like Cube Runner, Tris isn't too graphically intensive so it's battery safe as well.
Ever wanted to record a lecture, a meeting or a memo? If you don't want to get your wallet out, Voice Notes has you covered. There are better paid for apps, but Voice Notes gets the basics right. Starting and saving notes is easy and unlike at least one other free voice recorder, you can actually delete recordings! Unlike many similar apps you can also get your recordings off your iPhone, though it does require downloading a custom piece of software and as such isn't as intuitive as one would like it to be.
Not strictly an application, UK Trains is a custom designed iPhone webpage created by www.themacbox.co.uk that allows you to look up train times in the UK. Once found you can save journey details, bookmarking them to your Home Screen or having them emailed to you for future reference. It's quick, it's easy to use and it's a godsend when working on the move.
Planning ahead makes travelling painless and knowing what tube lines are actually working is part of that. TubeStatus gives regularly updated info on the status of all of London's tube lines, providing all the info on scheduled maintenance and incidents. An absolute must for anyone living in, working in or simply having fun in Britain's biggest city.