Here are some links I frequent (or otherwise really appreciate):

My Job	Cisco Systems, Inc.
SNMP MIBs	MIB Depot SNMPLink.org
SNMP Info	SimpleWeb.org The “Gearhead” Articles (Logging Messages with SNMP) The “Gearhead” Articles (The SNMP Show) The “Gearhead” Articles (The ABCs of MIB) The “Gearhead” Articles (Closing Our Traps – and messages)
RFCs	SNMP (RFC 1157) SNMP MIB-2 (RFC 1213) Structure of Management Information (SMI) (RFC 1155) Intro to SNMPv2c (RFC 1901) SNMPv2 Protocol Operations (RFC 1905)
Cool Tools	Net-SNMP OpenNMS Nagios Snort MTR SNMP4TPC (SNMP for the Public Community) GetIF Nmap SuperScan