The Xonar U3
It has a whole bunch of 3D audio enhancements aimed more for gamers, which I just disable through the device control panel, but if you are on a desktop or laptop with mediocre sound (or with badly isolated sound circuitry that transmits fan noise, etc.), this clears everything up for a reasonable price.
Also, since it is a separate audio path from the default system audio, you can dedicate it to audio player playback (I use foobar2000), thereby preventing computer alarm sounds and the like from interrupting your listening, or hurting your ears and/or headphones in the case you are listening to lower S/N recordings.
It also has a S/PDIF digital out in case you want to leave the digital conversion up to your stereo system. This is especially useful for laptops that only have analog outs.