Visual Studio comes with Dotfuscator Community Edition - a free tool to obfuscate .NET binaries. Its ok for lightly obfuscating your source code. Its not the best, but its free. Mmm, free stuff.
The gotcha is if a user sends you an exception with stack trace from your dotfuscated application, you need to purchase the full version of Dotfuscator to view it. DFStack is a cheaper option, but it wouldn't work consistently for me, and theres no source code available.
I ended up writing my own stack decoder (its free, and you can use the source code)