We use one called Xenocode, there's the free and commercial versions of
Dotfuscator, RemoteSoft have one (the websites are all name + .com), and
there are some more knocking around. They all have free trials, but the full
versions usually cost money.
You might also consider what you can add as extra value in terms of things
beyond the software itself - that makes buying your software more
attractive.
My two!
Steve