They released VampireTheMasquerade - AnneRice the RPG, but with options for kicking it up a notch on either the humour, the ass kicking or the angst. And they put in a novel and simple (and crap) system, which involved the ctarthis of using many many dice at once.
It was successful beyond their wildest dreams - and has been credited with sparking several years of renaissance in tabletop gaming.
(And rules light. Very rules light. No lookup tables here - emphasis on drama, with just enough stats to keep those with calculators happy)
There's DemonTheFallen? now too, or is that different? -- Senji
There's mummy too - they do a lot of extra stuff that's not technically a part of their main five.
They also released many many supplements, many many many add ons, and massively overpopulated their world with supernatural beasties. Which lead to them releasing a whole cycle of 'the humans fight back' They are currently working on re-releasing the big five set in historical times.
Their world hangs together, includes many well crafted and slightly disguised stereotypical elements and all hangs on a single metaplot.
Well, it hangs together if you ignore the fact that most games include world-spanning baddy conspiracies that really should be treading on each others' toes. Unless since I got bored with it they've revealled that they're actually, you know, all the same conspiracy.
Very much a product of its time, the nineties conspiracy/angst-fest. Very very very popular with adolescents as it combines simplistic politics (though some games are better than others) with the opportunity for spectacluarly broken power-gaming and munchinisation not seen since D&D: the rules practically have 'rape me' printed on each page.
Also creators of Exalted, which retains all of the same good and bad points of it's predecessors. Amplified, if you believe some. The world hangs together slightly better than previous efforts, due to a slight reduction in the conspiracy count as much as anything. Several legions of dice are still required (I have personally seen 31 rolled, and that was by a supposedly low-powered character...), and the rules are as eminently breakable as ever. Still, the universe is intriguing and provides lots of oppurtunity for general chaos to ensue.