Boston:

Grasshopper - Excellent food, low prices, 100% vegan.
Site: http://grasshoppervegan.com/

Cambridge:

Oleana - Slightly upscale restaurant, with an amazing 6-course vegetarian tasting menu for $40 a person - a total steal. The restaurant has a beautiful outdoor dining area - if you do choose to go there during the weekend and want to sit outside, make the earliest reservation (as of now, 5:30 PM), and get there around 5PM and wait in line. It's well worth it.
Site: http://www.oleanarestaurant.com/

If you are a hungry vegetarian/vegan, stay away from...

Radius - While considered one of the best restaurants in Boston, its "Vegetables 5 Ways" menu choice, while tasty, has remarkably tiny proportions. I appreciate fine dining, but to have a single piece of cauliflower constitute 1/5 of an already tiny meal - that's a little absurd. However, the service was amazing, and the non-veg people loved their food.
Site: http://www.radiusrestaurant.com/