I've just been given an old Win98 PC, and my first thought was to check out Linux, as I've never even seen it!
So I'm doing that at the moment, but it occured to me that some of the more experienced contributors to this forum might have some suggestions.
It's a 500 Mhz PIII with 128MB of RAM, and I have 2 other machines - a 1.7Ghz P4 and an AMD 64bit box. Before people suggest setting up a home media server (if that would be even possible), I live in a rented house and my house-mates might not be prepared to support that move, and I can't start laying cables all over the place even if they would (wouldn't be worth the effort anyway, as I may only be here for another year). That said, we do have a wireless network on a Linksys WRT54G, and my house-mates might be into it, even if it required some cabling and expense.
However, I'd also be interested in suggestions that weren't MC -related, so please let me know if you have made or know of good uses of aging, superfluous PCs...