Mackie HR-824's are excellent, got em for 1100 new thats what I use, it has adjustable cut off for 39,47,80hz, 1/4, 1/2 full space, frequency, etc. not everyone likes em. i produce drum n bass + garage so
it can be a bit exaggerated on the lows if i set it all the way down to 39hz, but any monitor that gets down to 45-60hz is good.
but i get very nice mixes on them, i used to use crappy ass Alesis Point 1's (only go down to 80hz)
My mastering engineer uses KRK's
Dynaudio BM6A also a good choice, tho its only 6.5\" woofer it goes down pretty low i know a lot of people love them
ADAM (havent really tested those out)
genelecs are nice, overprice though
it really comes down to personal taste. Take a CD of mastered tracks whether of your stuff or of the type of music you do and do a shootout at the local dealer. There are so many combinations of parts and approaches to studio monitors - i.e ribbon, fluid cooled ferric, silk tweeters, front bass ports, back bass ports, etc.
the other 1/2 of monitoring/referencing is acoustically treating your control room, otherwise it doesnt matter how eq'd or high/low end monitors you have, cause it will sound like crap in the car/boombox/pa.
Its a real art + science, im not an expert either, though i do talk with a friend who is an acoustic engineering whiz. i could go on, but ill let the other more experienced peeps here have their say.