Sell it. If you just want to be rid of the system, try selling it: Any of the popular auction sites are an obvious place to start.
Hand it down. An unmodified, unimproved older PC still can be fine for tasks like simple word processing or email. If there's someone in your organization or home who spends their time on non-compute intensive tasks, an older PC may be all they need.
Donate it. Whatever you do, don't throw away a still-working PC. If you have no use for it, call a local school, church, nursing home, hospital or charitable organization and see if they can use it. (On the other hand, don't pawn off junk under the guise of charity: If your system is broken, do the right thing and fix it before you give it away; and always let the recipient know the limitations of the machine you're giving them) You'll not only be doing some good; you also can take a tax deduction for the current value of the system you donate.
There you have it: 12 separate things you can do with an old PC. OK, now what are your ideas? Or, if you have questions about any of my suggestions, I'll do my best to answer them. Either way, join in the discussion!