http://sharpsnmplib.codeplex.com/ Including tests and even a Browser Recommended
There is another open source alternative, based SNMP++, the base is HP c++, the basic problem of this package is that is not fully ported to C#, but in the other hand has the power of c++, I tested this package a couple of years ago in a management product and works OK (ver1,2) Includes all that you need if performance is the issue and if at last productivity is a must this can be a problem. http://www.maom_onet.republika.pl/snmp/snmp_ppnet/ Also is based VS2005 and not updated!
Excellent work regarding SNMP v1,2,3 Take a look of Milans work http://www.snmpsharpnet.com/