I discovered this morning that gkrellm, a Linux system monitor which I've been using to keep watch on my new CPU's temperature, can remotely monitor other systems as well — and that there's a Windows port.
So I set up a bank of monitors to watch most of my machines, reporting CPU usage, processes, CPU/graphics card/hard drive temperatures (where supported by the motherboard), disk and ethernet traffic, memory usage and uptime:
I really like being able to keep an eye on my Web server, half a continent away, without having to ssh into it.