What are the best quality speakers for under $500?

Dahlquist DQ-20 and DQ-10.

a short list of used marvels...dahlquist 10,20....the large advent...spica 50,60....rogers studio 1....jbl 4312....there are tons more. all are different, and all but the spicas are great allrounders.

I heard the Silverline Minuets at NYC Stereo Show and they were very impressive to me and others in that room. They list for $600.

Spica 50s.

Kopenhagen,
It is really going to depend on a lot of other factors what are going to be the best sub $500 speakers in your system and your room. The "best" speakers are likely to be very transparent and reveal the shortcommings of amps and other components.... something we don't want when we are on a budget. So depending on the components that you have, it will really depend. I did help a friend out with a modest receiver, and upgraded his speakers to Wharfedale Diamond 9.1 ($350) with very pleasing results.... brought his system to a new level without exposing the shortcomings of other components. I would also look at the Epos ELS3 and offerings from NHT, but these may be less forgiving than the Wharfedales.
Good Luck,