When I was a student a group of us would go to the local auction house and look for bargains. At the very least it was entertaining to see what some people would sell and what others would buy.
After a while we started buying old bikes that were beyond their best. We'd then combine parts from the various metal carcasses to produce a functioning bike. If I'd applied myself to my studies with the same enthusiasm my grades would have been a lot higher.
Here are some reminders of those fun times:
The bicycle forest
Crazy home made bike mods
A gallery on flickr of users homemade bikes
So, before you consign that old bike to the rust heap try giving it a new lease of life by transplanting some new 'organs'. You may be surprised how much fun and how satisfying it can be.