People always want you to bring them back something. I don't like buying shirts that say 'I <3 NY' or 'BOSTON' on them...ok, I like the Boston one, but you get the point. I've found that the best souvenirs are personal things that you can actually relate to a person. Something that can 'only be found' in the place you're visiting.

Case in point: NYC Condoms, with the package design like the metro signs. Priceless and free, handed out by street peeps. Probably not the safest condoms; so I wouldn't recommend using them! Pictures are also good!