Broadway and off Broadway:

Most Broadway shows have shows at 2pm and 8pm on Saturdays.

Ticketmaster.com - [ www.ticketmaster.com ]
StubHub.com - [ www.stubhub.com ]
Broadway.com - [ www.broadway.com ]

A sample of some of the shows happening now:

Jersey Boys - The story of Frankie Valli and the Four Seasons.
Wicked - The untold story of the witches of Oz .
Gypsy -

Off-Broadway:

Blue Man Group