A good place to go for free SNMP software is netsnmp.org,
or maybe now it's net-snmp.org,
home of ucd-snmpd. The learning curve is steep, but the software does the job, and covers
both client and server side.
In January 2001, while working at Disappearing Inc. (now Omniva Policy Systems),
I needed to make some minor updates to the SNMP agent in our server. One thing
led to another, and soon I found I'd come up with a little helper library for
ucd-snmpd that might save other people some time. When I left Omniva in July 2001, they kindly
gave permission to open-source the library. I polished it up, named it 'fsubagent',
and published it here. Enjoy.