the audioquest dragon fly's are very good for the money.
IMO the Parasound ZDAC sounds a lot better than it should given the price. I don't have a lot of experience with dac's though.
Parasound zdac, Simaudio 100D, Peachtree Dac-itx - all under $200 used...
I agree with the recommendation of the Simaudio 100D. It's great sounding, exceptionally well built and a bargain at its used price.

I have one in my office system and am amazed by what it contributes to the realism of the sound from a computer based system.
Peachtree! Big Bang for the dollar!
I like the ifi micro idac2.  I don't think I paid a lot for it, and it will play DSD files natively.