Wireshark

This used to be called ethereal but was renamed to Wireshark in 2006. Sniffs packets from the network and deconstructs many network protocols. Works extremely well.

gkrellm

Provides a very nice stack of system monitors.

Items marked with (nyt) indicate I have not yet tested it. I've heard
it's good (so it has made it on the list) but have not yet tested it
myself.