The Atari 2600 has only two controller ports, it does not have a multitap. The Paddle Controllers come in a pair that connect to one port, however, so four people can play on one system. Few games support this — games with simultaneous multiplayer on the 2600 must be Paddle games, and must be multiplayer paddle games, to have 4-player support. In addition to games like Warlords with simultaneous multiplayer on the paddles, I will also list games which support 3+ player alternating multiplayer on normal controllers. The list of 2600 games with 3+ player support is short, but does include a few great games, most notably the all-time classic Warlords. Warlords is easily the best Breakout-style game on the 2600, and really is a must-play game!
Source: http://www.atariage.com/forums/index.php?showtopic=117531&st=0&p=1419114&#entry1419114
Key: Atari title/Sears title (for titles with alternate Sears titles)
Three Player Simultaneous w/ Paddles
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Blackjack
GI Joe Cobra Strike
Four Player Simultaneous w/ Paddles
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Breakout/Breakaway IV
Casino/Poker Plus
Medieval Mayhem (recent homebrew improved version of Warlords)
Party Mix
Street Racer/Speedway II
Steeplechase
Video Olympics/Pong Sports
Warlords
4-player alternating (with a single regular controller, usually)
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Communist Mutants from Space
Decathlon
Fireball
8-player alternating (with a single regular controller)
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Summer Games
Winter Games
California Games
This is incredibly informative. I wonder if I could volunteer a follow-up question?