I use Alesis M1 Active Mk2s and they are great !

You want to look for words like "flat response" (doesn't color the output to make it sound nicer NOT LIKE HEADPHONES !!!). This is very important when you are mixing as flat response monitors enable you to generate a mix that sounds as good as it can on as many different types of stereo as passable, from iPods to car stereos to big old home systems.

Near-field monitors are good to look for as you don't have to worry about the acoustics of the room too much. You set them up at eye level 2 to 3 ft apart from each other in a triangle with your head as the third point.

Bi-amplified is also good as it minimizes distortion from crossover.

Avoid high end headphones as they do several bad things as they make your mix sound nicer than it really is. I once mixed down a track with 3 or 4 vocal parts only to find ALL the harmonies were out when I listened to it on a boom box. Fortunately I was the only one listening to it !

Headphones also alter the spatial distribution of sounds giving a false stereo effect. This can also be very misleading.