A lot of weed and alcohol. People more curious than that will try LSD, schrooms, DMT, cocaine, Exctacy and MDMA. And pretty much any other drug they come across, at least once, except opiates. Never met anyone in IT doing opiates as far as I know (could be wrong). That is my experience amongst colleagues under 40 in the IT industry. Doesn't differ much from the other industry I've been in. None of these people have been effected by their weekend sins much besides hungovers and a thinner wallet.