Sure, Calexico and some of the Halal carts are great, but you would be hard pressed to find a block with a consecutive string of food stands to choose from, each specializing in a different dish.  It was hard to walk around Seoul without passing by a food cart (or several).  They run at all hours and are very reasonably priced - you can eat a satisfying meal for under $3.

Some tasty dishes you might find: ddukbokki (spicy rice cake), kimbap (rolls, like futomaki), soondae (Korean black sausage), teegim (breaded fried things, like tempura), hoddeok (honey-filled Korean pancake), boonguh bbang (hot-pressed red-bean pastries shaped like fish)

Here are some photos of carts that we came across in Sinchon (pronounced sheen-chon).  Taken "from the hip," so to speak.