I think I see a circular argument there…how are you going to develop an ear for such things as eq subtleties if you don’t get half-decent monitors? 
for low-budgets: the Alesis Monitor MKII are also ok … but these are passive speakers … in addition you need a high quality amplifer
I just bought mine (3 days ago). And I did a lot of research on monitors in the $500 range, and 3 names where mentioned constantly:
KRK Rockits RP8
Events TR8
Tapcos S8 by Mackie
Tested the RP8 (supposedly the best in this price category) against the TR8, and found the Events to be booming (bass not tight). The KRKs were nice and flat.
Tested the RP8 against the S8, and found that the Tapcos go lower on the bass. Absolutely no difference to my ears on the mids and higs.
So here they are in my setup, huge 8’’ monitors and they sound awesome!!! They were a little more expensive, but if I couldn’t go higher the $500 I’d get the KRK Rockits for sure.
PS: if you have the money to spend go for Mackie or Genelec.
Dynaudio Acoustics came out with a new model recently - the AIR BM5 I think (or something like that anyway), and I think it has a price tag similar to the Mackie HR824s, only it’s Dynaudio. 
I recently ordered a pair of Event Tr8’s. After checking out a great number of Nearfields I picked the Event’s. I also looked at the KRK’s RP8’, but those monitors only go down to 45hz, which is definately not enough for my style of music. If I were to get those, I have to spend an extra 300 euro on a subwoofer.
So I ended up with The Events, they can go down to 35 hz so I don’t need a subwoofer. They sound pretty flat, not perfect ofcourse, but really good for the money in their price range.
If you are on a very tight budget, try the Behringer Truth’s. I was very suprised how they sounded. They sounded better than the Alesis Monitor One MK2’s! And I am definately not a Behringer fan. (I had several gear which I later sold) But their Truth’s are quite good in the low budget range.