It is incredably frustrating that it is almost impossible to find decent Mexican food in Northern New England. Part of the problem has traditionally been that there isn't a very big Latino population here - though that is changing - but there hasn't been much decent, authentic Mexican around - usually tastes like cardboard, covered with melted cheddar.
There have been a couple of chains opening up in recent years, but they are much better at mixing margaritas than producing the actual food.
Two small, independent restaurants to keep your eye out for though:
Henneker, NH - Que Pasa. Small - maybe 10 tables. More TexMex than anything, but a nice, friendly atmosphere and hidden away in a little town in the middle of nowhere. The food is ok, but it is more fun for the sense of having found a hidden secret.
Much better - Saxtons River, Vermont - Golden Egg Diner. A small-town diner that specializes in breakfast (good ones with organic eggs). The chief cook however is a clasically trained Mexican chef who ended up in this small town somehow. Friday and Saturday nights are "Mexican Nights". The food is authentic and delicious. Sly (chef Sylvester Gaillegos) has actually found a source for fresh tortillas and the difference it makes is incredible.
Any other suggestions, anyone?