Dotfuscator Community Edition is 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 the full (expensive) version of Dotfuscator to view it. DFStack is a cheaper option, but it wouldn't work consistently for me.
I ended up writing my own stack decoder (its free, and you can use the source code)