TSR, Inc. , p.n. (formerly) Tactical Studies Rules Inc.; a Lake Geneva, Wisconsin-based publisher of hobbyist games and fantasy literature, founded in 1974 by E. Gary Gygax; publishers of the Advanced Dungeons and Dragons, Boot Hill, and Gamma World RPGs, amongst many other products; acquired by Wizards of the Coast in 1997. [GJ1]

Addendum: For a history of TSR, see the addenda for Dungeons & Dragons. [GJ1]
Addendum #2: Before the downturn of the company, it was often pejoratively referred to as T$R by gamers who felt that the company was just producing supplements only to make money. Now, of course, they look fondly back on TSR as they think that WotC/Hasbro are just producing supplements (and new editions) only to make money.