The staff at B's Bikes is always generous with their time and expertise even when you're a complete cycling newbie.
I got one of my road bikes -- a Fuji -- here, and I've been pretty happy with its efficiency. The bike came with a service warranty, and the staff is really helpful with tune-ups. Before I got my pump, I went here to fill up on a regular basis as it was around the corner from my house, and they left a pump outside for the community to use. Definitely the community bike store for Greenpoint.
A local favorite for all of Williamsburg, with an excellent staff and selection and prices as reasonable as can be. The staff will bend over backwards to troubleshoot your cycling experience, get you on the right bike or fix the one you have, and they've been known to keep the store open if a problem has yet to be solved. It's rare for a customer to walk out unhappy.