Stealth Inventory 2.2 (by Loadbalancer.org Limited)
Stealth Inventory provides a small 100k agent that runs invisibly

Netoscope 1.6 (by Basta Computing, Inc.)
Netoscope: Internet server monitor.

The Dude 1.1 (by MikroTik)
The Dude will layout your network on a map and will monitor services of devices

Sniff-em Packet sniffer 1.12 (by YASC LTD)
Sniff'em 1.12 is a Windows based allround Packet sniffer and Network analyzer.

SNMPTest 1.03 (by Noel Danjou)
Query and monitor any SNMP-compatible device

MailMonitor 1.6.3 (by DigiArch.org)
If you have mail problems, MailMonitor shows you what's going on at port level.