Corner Billiards is also quite decent. It's on the corner of 11th St and 4th Avenue. It's not that cheap but the equipment and the atmosphere are nice.
Broadway Billiards is located at 10 E. 21st St (Broadway @ 21st St.). It's without doubt the cheapest place to play and the easiest to find small action. $4/hr during non-peak hours, and $5/hr during peak. The tables are being re-done right now, so some of them even have brand new felt on them. Otherwise, the equipment is just so-so. It's located down stairs in the basement of the building.
SoHo Billiards is crap. Located on Houston just east of Mulberry St. The tables roll off and most of the ball returns are hanging loose from the tables so the balls actually drop out once they fall into the return area. Very annoying. Stick to Corner or Broadway.
As the poster before me mentioned, there's Amsterdam. A little on the pricey side, but home to a lot of pros (both men and women). You'll probably bump into Tony Robles, Hohmann at the moment or Jennifer Barretta if you stay there long enough. Good equipment, good atmosphere. Located on Amsterdam @ 77th St.