I was going to do a guide for DUMBO but there really isn't a heck of a lot there now. There's Smack Mellon, 92 Plymouth Street, and the DUMBO Arts Center around the corner at 30 Washington Street -- both great -- and about nine or ten so-so galleries on the second floor of 111 Front Street. That's pretty much it. Many galleries closed. (I heard about one that closed because they were robbed too many times -- and they were on the ELEVENTH FLOOR!) Many others are really design or clothing stores.
Of course, aside from Smack Mellon and the DUMBO arts Center, it's worth going to DUMBO for some of the most dramatic urban views you'll ever see.