Best small, desktop speakers, $500 or under (new or used)

Another vote for Vanatoo Transparent Zero, incredible speakers from a very responsive company. You must use their interconnect between speakers, it is NOT a regular ethernet cable. Excellent sound from a tiny set of speakers.

Thanks for the many additional ideas. I like the idea of a very efficient speaker, if only because I’d be curious to try driving it with a small tube amp someday rather than my Adcom. I agree about the Atoms. Dull. The JBL and that Vanatoos still sound interesting. There really is too much out there!

These are too much, but the seem like a good, more expensive desktop option— Decware Trapeziums 

Audioengine A5 + Audioengine S8 subwoofer......used.....

I also recommend Audioengine 2+ speakers for true desktop use. Really nice performance for an inexpensive price. Heck, I even got out of my "speakers should be heard and not seen" mode and bought the red ones (really attractive!)

Assuming you are not looking for powered speakers such as AUDIOENGINE, the ELAC B 6.2 currently at $244 /pr. is pretty hard to beat.

I just jumped on these yesterday on Music Direct, matter of fact.  I had been looking at B stock units on Elac America for $209 but got sidelined by a small flooding issue here in mid-Michigan last week.  When I got back around to pulling the trigger on them this week, they were sold out.  A-stock was $349, then yesterday every carrier of the brand seemed to drop the price to $244.  So I pulled.  Looking forward to putting them in the office next week.