Taco Del Sol has won the best lunch for under $6 vote every year that I've lived here. Their burritos are fantastic, but a little known secret is that their
tacos are a good meal size and much cheaper.
The El Cazador truck is great late at night. The food is really flavorful and you get more than you would expect for the price.
Front Street Noodles and Wraps is another place to fill up on good food for cheap. I also like that you can customize your own bowls.
I haven't eaten at the Catalyst since it has changes owners, but they used to have a pretty good soups or sandwiches for around $5.