My vote is for Little Dom's 15$ Monday night three-course dinner. Had it last Monday and it was awesome. Tomato bruschetta (these were weak--not enough tomato), then excellent seafood risotto and plum crumble cake. Excellent food at any price.
I also like Park in Echo Park (3-course prix fixe on Tuesday for 15; any 3 courses off the menu on Fridays for 20)--really solid food, though not quite as good as Little Dom's. And I like Zanzabelle's Freight by Night (Fridays and Saturdays; 20-30 for 3 courses); it's cooking is a little hit and miss, though fairly ambitious. Haven't tried Canelle yet in Atwater.
What's your current favorite? Low price prix fixe menus really help me eat out in the current economy.