To compliment the exisiting [Essentials] thread, I would like to add this [Mulitplayer Essentials] thread as a list of the best games to play with a friend or online.
Multiplayer Essentials
Star Fox Command (WFC)
Mario Kart DS (WFC)
Tetris DS (WFC)
Club House Games
Metroid Prime: Hunters (WFC)
Elite Beat Agents
Advance Wars: Dual Strike
Bomberman / Bomberman Land Touch! (WFC)
New Super Mario Bros.
Diddy Kong Racing (WFC)
Worms Open Warfare 2 (WFC)
Bleach DS / Second (WFC) [JP]
Mario Party DS
Multiplayer Notables
Ultimate Mortal Kombat (WFC)
Moero! Nekketsu Rhythm Damashii Osu! Tatakae! Ouendan 2 [JP]
Madden 08 (WFC)
FIFA 08 (WFC)
Contra 4
Glory Days 2
Brothers in Arms
Need for Speed: Pro Street
Top Gun
Super Monkey Ball: Touch & Roll
Namco Museum
Tingle no Balloon Fight [JP]
Jump Super(Ultimate)Stars (WFC) [JP]
Yu-Gi-Oh! GX Spirit Caller (WFC)
Yu-Gi-Oh! World Championship 2007/2008 (WFC)
Metroid Prime: Pinball
Big Brain Academy
Daigasso! Band Bros. [JP]
Yoshi Touch and Go!
CastleVania: Portrait of Ruin (WFC)
Bubble Bobble Revolution (Classic Mode)
Children of Mana
Quake DS (Homebrew)
Tank Beat
Custom Robo Arena (WFC)
LostMagic (WFC)
Animal Crossing WW
Pokemon Diamond/Pearl (WFC)
Multiplayer Essentials
Star Fox Command (WFC)
Mario Kart DS (WFC)
Tetris DS (WFC)
Club House Games
Metroid Prime: Hunters (WFC)
Elite Beat Agents
Advance Wars: Dual Strike
Bomberman / Bomberman Land Touch! (WFC)
New Super Mario Bros.
Diddy Kong Racing (WFC)
Worms Open Warfare 2 (WFC)
Bleach DS / Second (WFC) [JP]
Mario Party DS
Multiplayer Notables
Ultimate Mortal Kombat (WFC)
Moero! Nekketsu Rhythm Damashii Osu! Tatakae! Ouendan 2 [JP]
Madden 08 (WFC)
FIFA 08 (WFC)
Contra 4
Glory Days 2
Brothers in Arms
Need for Speed: Pro Street
Top Gun
Super Monkey Ball: Touch & Roll
Namco Museum
Tingle no Balloon Fight [JP]
Jump Super(Ultimate)Stars (WFC) [JP]
Yu-Gi-Oh! GX Spirit Caller (WFC)
Yu-Gi-Oh! World Championship 2007/2008 (WFC)
Metroid Prime: Pinball
Big Brain Academy
Daigasso! Band Bros. [JP]
Yoshi Touch and Go!
CastleVania: Portrait of Ruin (WFC)
Bubble Bobble Revolution (Classic Mode)
Children of Mana
Quake DS (Homebrew)
Tank Beat
Custom Robo Arena (WFC)
LostMagic (WFC)
Animal Crossing WW
Pokemon Diamond/Pearl (WFC)