http://www.taxi-van.co.uk/ - hire a man with a van for £30/hr
http://www.streetcar.co.uk - hire cars for £6/hr (plus £50 joining fee). One is in Park Street Car Park, right next to the ADC. Stuart Moore (stjm2) is a member.
Practical Car & Van Hire Cambridge - based on Cowley Road, near the Science Park. Brian is friendly to students and unlike Europcar, National and Thrifty won't necessarily sting you with inflated repair costs for minor bumps and scrapes! Note that Practical only hire vans to over 23s due to the terms agreed with their insurers; this may be negotiable with sufficient advance notice.