www.prospect.ac.uk is the main site I've used - Graduate Prospects and Career Information.
You can search for job types and postgraduate degree opportunities. Each job/career has information regarding qualifications needed, nature of the job, typical salaries, case studies of people who work in that area, career opportunities for the future (i.e. promotions, predicted salary after so many years in the job) etc.
It's a pretty good site
.