The West Side Lounge on Mass. Ave between Harvard and Porter is fast becoming my favorite brunch place in the area. Everything is $6-9 and the food is of much higher quality than at dinner.
My favorite is the Monte Cristo - a homemade croissant breaded and stuffed with ham and swiss cheese, served with raspberry maple syrup, fried potatoes and fresh fruit. The croissant layers are delicious if a bit soggy, but the contrast between the crisp breading and the smoky, savory thick pieces of ham is just astounding. The fried potatoes are perfect; some of them are really soft sweet potatoes, which is a nice touch. The breakfast tacos are also great - flour tortillas filled with scrambled eggs and sausage, topped with an acidic tomato/cilantro/onion salsa and the same accompaniments. The orange brioche french toast with Jack Daniel's butter is good for thick french toast, although I'd like a bit more of the egg bath... great for $7 though. I haven't tried any of the sides, they range from fried eggs and english muffins and stuff to meats and smoked salmon with Important-Sounding Farm designations, which I assume must be pretty good based on the quality of the ham. The alcoholic beverages also look really good, haven't tried them yet.
The atmosphere is even better - it's never full, there's good music, the booths are plush and comfortable and the room is full of natural light. You can just sip coffee and read the paper as long as you like. The staff is very genial and attentive. I've been there 3 weeks in a row, and I will continue to go every week I'm in Cambridge.