The dev team I'm with has just adopted the Microsoft Enterprise Library Blocks for use with .NET. They have a logging section that may prove very useful in testing. I've just started with it so can't make any recommendations on how to implement it. Find the walkthroughs and examples on MSDN. They are very helpful and show just how easy this is to use.
The EL isn't bad, as it means less code that you have to write. And since you get the source, you can modify as necessary. The downside is that you have additional binaries to deal with, as well as a handful of config files. If you only need a small piece of the library, I'd recommend creating your own library, as the overhead of the EL wouldn't be worth it.