There are many protection system for net, so called "Obfuscators",out there:

-Demeanor
http://wxw.wiseowl.com/

-Salamander
http://wxw.remotesoft.com/salamander/
Protetion through Translation

- IL-Obfuscator
http://wxw.9rays.net/cgi-bin/components.cgi?act=1&cid=86

- Dotfuscator
http://wxw.preemptive.com/products/dotfuscator/

-IL-Obfuscator
http://wxw.lesser-software.com/en/content/products/LSW%20DotNet-Tools/LSW_DotNet_IL-Obfuscator.htm


- Deploy.NET

http://wxw.junglecreatures.com/DesktopDefault.aspx

- xenoCode Enterprise

http://wxw.xenocode.com/en/Product-Features.aspx

-Dotfuscator Community Edition

http://wxw.preemptive.com/obfuscator.html


and many more