There's lots down there and the staff are very helpful - link to their website below but scroll down for a list and further information.
Don't forget to take some i.d. as you'll be photographed and issued with a pass to get into the actual search room. The refreshments on site are restricted to a couple of vending machines.http://www.glamro.gov.uk/navigating.html
Records include Courts, including Quarter Sessions, Hospitals, National Coal Board Registers of Shipping and Crew Lists Property Valuation Lists
Glamorgan parish registers most now on their computers, nonconformist registers, Glamorgan FHS indexes to christenings, marriages, burials, monumental incriptions.