Bartley's Burger Cottage is a Cambridge tradition. It's a hole-in-the-wall, crowded, student sort of place with a huge variety (40? 60? 80? different kinds) of hamburgers. There is other food available, but you go there for a hamburger. Bartley's is not your standard, assembly-line hamburger shop -- it's a one-of-a-kind. No reservations, of course.
Bertucci's is a very high-end pizza and general Italian restaurant. (It's not gourmet Italian; it's gourmet pizza.) Excellent food, reasonable prices, great bread, usually crowded. It's a Boston-area chain with probably a dozen restaurants total. This location is smaller than the Bertucci's in Central Square.
Café Sushi is a mostly-sushi restaurant which is a favorite place for many local software developers. (I can't speak to the quality of the bait sold there myself, but many of my colleagues like it a lot.) Entrees are in the $12-$18 range.
Not as inventive or high quality as Legal, but substantially cheaper and less crowded (and closer to the Smofcon hotel).
I haven't eaten at the Harvest in a few years -- the salesman who used to take customers and the occasional engineer there is long departed -- but it used to be and probably still is one of the better restaurants in Harvard Square. Reservations required.
See under Allston/Brighton; this one is in a former bank, with seating in the vault.
This is the only microbrewery in Harvard Square. The beer and food are average or better. It's in the basement of the building which houses Pandemonium, the SF bookstore.
Somewhat Americanized middle Eastern food plus a variety of burgers, teriyaki, steak, etc. You can get kabobs, falafel, hummos, shawarma (recommended), salads. They have takeout and vegetarian meals.
The Hasty Pudding is a Harvard student club with theatrical pretensions and a great deal of history. (Five US presidents were members in their student days, for example.) Nowadays, it does some theater, but as far as I know its days as a student dining club are done.
Upstairs at the Pudding is an excellent restaurant located on the top floor of the Hasty Pudding. It's once of the nicest restaurants in Harvard Square. (It has a great roof-top dining area, too, but that is probably only of theoretical interest in December.) The cuisine is non-ethnic and fairly eclectic, but everything is superb. (For example, they served the best Caesar salad I've ever eaten, bar none.) I've eaten there are lunch time on business, but never at dinner. Lunches can easily run up to $20 or $25, and I would expect dinners to be correspondingly expensive.
Reservations are mandatory.
Allston/Brighton * Harvard Square * Central Square * Near MIT * Elsewhere in Boston