If it hasn't already, Doom 2 should win the award for the most modified (and still played) video game ever made. Quite a few of the player-created levels are impressive and fun to play, but not many are made by a lead programmer of another popular game series. Instead of taking a well-deserved rest after the release of Bioshock 2, JP LeBreton (creative force behind the Arcadia and the Farmer's Market areas of the second installment of Bioshock) has re-created Rapture's Arcadia in an impressive level of the ancient Id shooter.
LeBreton used a modern-day Doom 2 editor called SLADE to "demake" this level. You can download this great tribute to two familiar game franchises here for free.