The release of Doom 3: BFG saw the disappearance of the original Doom 3 from Steam, notable for two reasons: 1) the game was rather cheap and 2) there were a bunch of mods for it. People were rightly pissed, Bethesda Softworks offered a hurried non-answer, and the whole thing looked like a cynical sales-protection strategy.
Well, OK, fine, you win. Doom 3, the original, is back on Steam. For $10. Specifically, here's the pricing:
• Doom 3 Pack: Discounted at $12.99
• Doom 3: Discounted at $9.99
• Resurrection of Evil: Discounted at $4.99
Further, Bethesda says Doom 3: BFG Edition's GPL source will be released "in the near future."
Original DOOM 3 back on Steam [Bethesda Blog]