CodeVeil

With CodeVeil you can encrypt your .NET applications and DLLs. CodeVeil adds native code to the assembly to decrypt the assembly just before the .NET runtime needs access to the data. The decoder includes anti-debugging, anti-tracing, modification tracking, and various other advanced techniques to prevent unauthorized access and modification of your assemblies. And, veiled assemblies can be used just like any other assembly. No third party dll is required with your distribution.

Requirements:

Windows 2000/XP/2003 Server/Vista, NET. Framework 2

Limitations:

30-day trial, nag screen on protected assemblies