You can run a decent linux system on just about any old piece of junk. It's certainly worth doing, if only just to play around with and get a feel for how it works. It's great for developing and testing since downloads and uploads are nearly instantaneous over the local network.
It's also great for hosting low traffic sites. Many low end service packages have a no-server policy, but I have never had complaints on sites that only get a few dozen hits a day. If you have a lot of these you can save some money. Beware the cost of powering a server 24 hours a day though. I recently moved a server from one location to another. The electricity bill dropped ~US$50 in one and increased by the same amount in the other. An older home workstation probably consumes less, but it still uses electricity.