I had seen this application in the AppStore before we left for London but for whatever reason I didn't think it would be needed or all that useful. I installed it a couple of days after getting here and have used it a lot. If you find yourself needing to convert from one thing to another a lot Units is the application for you. Currencies, lengths, speeds, etc. — it does them all.
That isn't enough magic to get me really excited about an application. What I like so much is how fast it is to use. It uses a calculator interface to make it super simple to get a conversion done. I had an initial complaint with how long it took to find the right mode, but they fixed this with a user-interface change in the newest release as well as some additional conversions.
Anyway, another great example of a very handy mobile application that adds significant value easily.