ZeroconfServices is a .NET library that wraps around Apple's DNS-SD DLL for mDNS discovery and publishication.
The exposes an interface very similar to NSNetService and NSNetServiceBrowser from Apple's Cocoa library. It is easy to use, well documented and drops right into any .NET application.
The library depends on dnssd.dll, which is availble from Apple.
If you have any questions, please feel free to contact David Hammerton at http://crazney.net/