Don't abandon your bike when you leave campus! Instead donate your bicycle to a local organization. Wheels for Winners is an earn-a-bike program that gives bikes to children who earn them.
Local bike auctions are a great (and inexpensive) way to find yourself a trusty commuter or recreational-use bicycle. Bicycles that have been abandoned in the City of Madison are sold through a website. Bicycles are sold as-is and may need repair before riding. See also the police auction website.
The University of Wisconsin also recycles old bikes collected on campus at its Surplus With a Purpose (SWAP) Store. The SWAP website includes a list and description of the items available throughout the year. Additional ideas for acquiring low cost bicycles are availabe on the UW Low Cost Bicycle Handout.
Keep your bike maintained so you can keep riding.
Enjoy this bike maintenance video produced by the Wisconsin State Journal: