If you are not familiar with ideone.com, it is an excellent website that lets you compile and run bits of code in any programming language under the sun. Nontas Rontogiannis, a tech enthusiast from r2squared.net and student at Univ. of Patras has released a client for ideone on Windows Phone called ide7.
If you quickly have to show your friend the validity of your idea at a bus stop, this is an excellent app to have. I tested it with C# and C++ and works flawlessly. There are a couple things to remember, though. It is almost always a good idea to first load the sample program. For example, in my first run with the app, I forgot the using System; declaration in my C# code, and got an error message.
There are a couple of problems with the keyboard, though, something that I hope can be fixed in an update. First, the keyboard has spelling correction turned on, which means i is autocorrected to the capital letter I, while is corrected to While, which makes typing code a pain. Second, the syntax-colored text box does not let you place the cursor very accurately. Specifically, long-pressing on the screen does not cause a cursor to appear, which would have let your move it around and place it accurately.
ide7 is a useful application, and I had fun with it. You can find the paid / trial version here, and the free / lite version (that supports C, C++, Java and C#) here.