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 larger than the Bertucci's in Harvard Square.
Legal Sea Foods is a chain of about ten restaurants which give consistently the best seafood in Boston. Prices tend to be high, the restaurants are always crowded (Their web page says they take reservations - or expect an hour's wait at peak times). Entrees run $10 -- $20.
The Royal East is one of the two best Chinese restaurants I know of in Cambridge. It's large, reasonably priced and seems to attract quite a few Chinese students from MIT which is nearby. If you're looking for Chinese, this is where I'd recommend. Parking is difficult, since the only parking nearby is on-street, and there's not enough of it.
Undoubtedly the most inventive ice cream make around Boston; wonderfully dense base, intense chocolate and a rotating assortment of flavors (Gingersnap Molasses is a classic) including sorbets. A few other desserts, such as creme "brulee" with a torch.
(The Student Center is worth a pilgrimage to see the MITSFS, owners of the largest SF library in the known Universe. Yes, there are several larger collections, but they're private; MITSFS hours are frequent if irregular (mostly evenings).)
Allston/Brighton * Harvard Square * Central Square * Near MIT * Elsewhere in Boston