Taco truck on the corner of 96 and Broadway. Got recently written up in the NY Times dining section by Peter Meehan: http://events.nytimes.com/2007/05/16/dining/reviews/16unde.html.
I also am a big fan of the al pastor tacos (there are real chunks of pineapple and onion that marinate with the pork!) at Noche Mexicana on Amsterdam around 103 on the West Side.
And of course, if you're willing to go Red Hook Ballfields-- those are the definitely worth the trip. There are food stands there that cover Mexican, Salvadorean, Dominican, etc. Written up here: http://events.nytimes.com/2006/08/23/...