Although it's probably the case that the hobbyist developer needn't worry too much about his .NET CIL code being decompiled, there are some free assembly obfuscators that seem to work: OrangeHeap (~ 1.35 Mb download) and Confuser (~2.2 Mb download; available here). I prefer the latter.

Tools like these have a way of becoming vaporware. Nevertheless, at the time I write this, you can find OrangeHeap here on the internet, and Confuser here.