Focussing on inexpensive ... For very good and inexpensive Vietnamese, try Pho Pasteur in Harvard Square (in the building called "the garage" on Dunster street). Also in Harvard Square is Bartley's burgers. If you have a yen for a really sinful, giant burger, this one's worth it. If you like informal Japanese, try the japanese food court in Porter Square (In the Porter Exchange building). You can get good Ramen there, served up on shared tables. I've heard the sushi is on the decline. Other options are available.