MF Gallery is a fairly new gallery on the Lower East Side that features lowbrow/underground/alternative art. It's a smallish space, but the owners (Martina and Frank, very nice, welcoming people) never skimp on the art. Shows feature top names in New York underground art (John John Jesse has had pieces in several shows) as well as more up-and-coming names of equal quality. One show featured the art of one of the musicians from the band Gwar, with the artist himself "entombed in an isochamber in the middle of the gallery" for a week.
If you're not familiar with the genre, be forewarned that some of the pieces can be distubing, but the craft of the work is always top notch.
The openings are always very low key and comfortable, usually with a limited bar or a keg (for donations, but you will be carded), but stop by anytime and take a look around.
MF was featured in the West coast based art magazine Juxtapoz as one of the best new galleries shortly after it opened. It's definitely a find in a city full of galleries full of uncrafted, boring work.