That's the way to do it. Writing a small utility app in C# for work. On Windows, of course, under .NET 3.5. Get the app running nicely and, just for interest, realise it ought to run on Mono (that's the binary, not recompiling the source).
So, copy the app to a Linux box, and, whoa, it runs perfectly under mono. No recompile, no downloads. It just runs. Perfectly.
By the way, I use SharpDevelop. It's neat, small, fast and free.