Duke Nukem 3D is finally available on Steam, and it's in a special Megaton edition, including almost all of the content released in the game's history. The expansions Duke Caribbean: Life's a Beach, Duke It Out in DC, and Duke: Nuclear Winter are all included in this version, as is the original game and the Atomic Edition, released back in 1996 with a fourth episode for the game, new enemies, new boss fights, and multiplayer matches against bots.
All of this is now available together on Steam for just $9.99 (currently on a launch sale of just $8.99). The game is even SteamPlay-enabled, which means you can install it on both a PC and a Mac. There's no better way to play one of the wackiest, best-loved first-person shooters of all time.