Black Box Improv Festival
Baltimore Improv Festival
Boston Improv Festival
Chicago Improv Festival
Comedy Improv Throw Down
Del Close Marathon
Improvaganza
Ladies are Funny Festival
Los Angeles Improv Festival
New York Musical Improv Festival
North Carolina Comedy Arts Festival
Phoenix Improv Festival
Providence Improv Festival
Richmond Improv Festival
San Francisco Improv Festival
Seattle Festival of Improv Theater
Snubfest Comedy Festival
Spontaneous Combustion
Toronto Improv Festival
Twin Cities Improv Festival
There are about 8,001 various comedy festivals throughout the country. For a detailed and ever growing list, visit the upcoming festivals page on the IRC Wiki.

Here are a number of notable festivals that Philadelphia groups regularly play, or that we can personally recommend should make your hit list:
Philadelphia Improv Festival
Nov. 1-7, 2010
(Comedy Month)
Philly Sketchfest
Nov. 8-14, 2010
(Comedy Month)
Duofest
Sep. 30 - Oct. 3, 2010
(Philly Improv Theater)
Philadelphia is currently home to three comedy festivals: