Remove Dungeons and Dragons from the confines of your parent's basement and spread it out across the entire city. LARPing (Live Action Role Playing) has a heavy focus on impromptu acting and story telling. While in game you assume the role of your character and respond accordingly to the actions of other players.
For more information, please visit the wikipedia article on LARP.
Rochester Rock-Paper-Scissors LARPS
Revelations - Vampire and Werewolf themed (1998-2004)
Black Sands - World of Darkness (1997-2000)
Rochester by Night - Camarilla Vampire themed (1995-1999?)
Vaulderie in Rochester - Sabbat Vampire themed (1996-2000?)
One World by Night (2000-2002)
One Last Breath - Vampire themed (2002-2004)
Eclipse: The Sabbat Revolt - Vampire themed (2006-2007)
"Dan's Game" - Sabbat Vampire themed (2007-Present)
The Camarilla - Multi-Venue New Mind's Eye Theatre Themed (2005-Present)
The Camarilla was also Old Mind's Eye Theatre (~2000-2005)
Rochester Boffer LARPS
NERO - Fantasy themed (2000-2008)
Winterfell (Dagorhir) - Fantasy Combat Sport, practices at Cobbs Hill Park on Monday nights, starting at 7pm, weather permitting.
Dagorhir focuses on full contact team combat, fighting with padded weapons, with no magic system to speak of.
Caradoc Hold (Amtgard) - Caradoc Hold, a chapter of Amtgard meets at Highland Park (near the veterans memorial, at 1pm) weekly.
Amtgard is a somewhat traditional LARP with classes, spells and a full magic system.
Rochester Live-Weaponry LARPS (and related events/groups)
SCA - Re-enactment themed (early 90's? -Present) (not sure if this counts, but certainly LARP-ish)
Rochester Free Form LARPS
N/A (to the best of EugeneCain's recollection)
Worth the Drive
there are LARPs in other cities, Ithaca, Syracuse, Buffalo, Geneseo, Brockport, and Oswego all have LARP scenes
Kingdoms of Novitas - fantasy themed boffer LARP in Central New York, with a large and growing membership (2004-Present)
FLAG - in Ithaca. High-Fantasy based LARP. Boffer style, ages 14+