Originally Posted by Beerman
A lot. Various mild and stainless steels; 4130, 4140, and 4150 chromoly; True Temper OX Platinum, Verus, and Supertherm (all modified from 4130); T45 carbon steel; 831 Reynolds; Columbus Nivachrome (modified from 4130, if I remember correctly); 6061, 6013, 7005, and 7075 aluminum; various grades of titanium; and, that's just off the top of my head.
The only stainless steel used in frames I *believe* at this time is Reynolds 953, there was a big deal about it at the last Interbike. BMX bikes are pretty boring...their budget for R&D and exotic materials tends to be much lower than the wallets wielded by roadies. True Temper, Reynolds and Columbus all have extensive *lines* of various chromium / molybdenum steel alloys (Columbus Nivachrome is actually alloyed with niobium, among other things), sporting various additional alloying agents and butting. Aluminum in the 6000 and 7000 series are occasionally seen in BMX (magnesium/silicon and zinc, respectively) but more exotic aluminum alloys exist in the roadie world, commonly 9000 series (zirconium) and scandium aluminum to name a few examples.
The big 3 are steel, aluminum and titanium, with carbon making up a nonmetal 4th entry. However within each category there exists HUGE variety.