Windows folks use P3's for door stops. We add big drives and use them as servers My ftp server is a Pentium 166. It works.
Back in 1997 or 1998, my uncle had a computer that was "dead". He gave it to me. It was a P120 with 64 MB of ram (roughly the same power as my laptop). It turned out that the 810 MB IDE drive w/ Windows on it had died. But there was a 4 GB SCSI drive that was working perfectly! I piddled with it in college, with Debian on it. I actually set it up as a "router" -- ran my ethernet connection from the wall into that, and then from the computer into my laptop. Now I use it as a "staging server" for websites I build -- I ssh into it from this computer, code, and upload to my web host. Works pretty well! I'm planning to leave all of the web stuff on there... it handles it just fine, and I don't want to bother with transferring everything.
My laptop from college died about a year after I graduated. I got this computer, and a few years later, bought a HD for the laptop on ebay for $20. Put Linux on it, and it worked enough for me to take on trips where I wanted internet access
I even got wifi working, which was a real chore w/ the card I had. Currently it's giving me some major issues, and every time I open it up, it seems the case cracks get larger, but I haven't given up hope just yet...