You might check floppyfw. It's a router / firewall that fits on a single floppy. Here's the link:
floppyfw . Note: you need two NIC cards in this box for this to work.
You could also:
build a standalone MP3 player / file server. Checkout this article from techtv:
MP3 player
use it as a backup server (if it has the hard drive space). You can image your other systems and copy the images to this machine.
Build a Beowulf Cluster (sounds hard, but might be worth trying, if you have the spare time): Link:
Building a Beowulf Cluster Link 2:
About Beowulf Clusters
Use it for home automation. The ActiveHome kit from x-10 only requires a 386 or higher and win 3.1 or higher. The software is about 50 bucks, and can control your lights and appliances, etc.