From Homestar Runner Wiki
Clapping Party is a video game, originating from the Strong Bad Email slumber party. Its objective is to cause two on-screen hands to clap by clicking the text "PRESS HERE TO CLAP". After clapping three times, a screen will appear that says "ROUND CLEAR!!". On the second round, clapping three times unlocks Blistergeist mode. This involves a ghost in a hockey mask attacking the hands with a chainsaw, causing large blisters to appear on them.
Clapping Party is the only video game Homestar Runner is allowed to play.
To play it, click on the "questionable" Easter egg at the end of slumber party.