I've personally used Adventnet's .NET SNMP API for snmp work. It's now been renamed to WebNMS. I have code running based on this API several places, that has just worked and keeps on working 24/7.
Recommended for lots of examples and stability. Also it's fast. Seems though that there are several other .NET SNMP libraries that have come since I used SNMP last that might be worth checking out. ex: #SNMP, which has been referenced to in other replies.
