Border Cafe in Harvard Square is great if you want a cheap definitely not fancy tex-mex meal. I love Harvard Square, and I always end up eating here. Obviously not gourmet or on pace with the rest of the thread in that regard, but def. enjoyable.
for a little higher end southwestern food, I love
Masa
I've gone there a couple times this year, and they have a tuna dish that is great, as well as a "Masarita" which is made with grapefruit juice.
They have an awesome/cheap brunch on sundays as well with texas toast w/ carmelized bananas and chocolate chip pancakes with mango butter.
I need to get out to more places, but these are two places where you're guaranteed to have good food and a good time.