In addition to the solutions provided by our commercial partners we can recommend the following free, open source SNMP software:
OpenNMS - Network Management application platform
SNMP Informant - SNMP Management of Windows OS - Collect low-level, mission critical performance data.
SNMP4J - Open Source SNMP API for Java
AGENT++ - C++ based SNMP Software, both agent and management side
Net-SNMP - Suite of SNMP v1, v2c and v3 applications for IPv4 & IPv6