Sutton Trust Summer SchoolWatch
They involve lectures (and practicals if appropriate) in the subject, staying at the university and experiencing some of the facilities and generally having a good time while you're there. Plus you get advice on applying too (especially useful for Oxbridge)
I did one of these at Cambridge for Physics, had a great time and went on to get an offer at Trinity for Nat Sci, although some got rejected.
They are available to everyone, but priority is given to those who go to poor schools with few applications to that university, have good GCSE grades and whose parents didn't go to uni/ don't have proffesional jobs. It's really worth applying, so if you are in Year 12 ask your head of 6th form (or equivalent) and have a look here: http://www.suttontrust.com/university.htm
This hasn't been updated for 2004 yet, but it should be shortly.
Which summer school did you go on grey faerie?