Mamma Maria - despite the cheesy name - offers top-notch Italian in the North End - and if you call ahead, you can reserve a private dining room! Way to knock ladies off their feet.
Just off Coolidge Corner in Brookline, by the Coolidge Cor. Theater is a little place called the Bodega Fiorentina with the best pasta I've had on this continent.
On Newbury St. just by Dartmouth, near Copley Sq., is a sushi joint called Shino Express with incredibly good sushi at remarkably low prices - no matter how much you eat, it probably won't go over $20/person AT THE VERY MOST.
+1 on Elephant Walk.
Anna's Taqueria is always worth a visit as a student mecca with good cheap Mexican.
Bartley's is the quintessential burger joint that lives up to its reputation.
I hear Om is very good, but I've never had the $$$ to go.