I'm glad to see a thread like this here, and stickied no less. I searched for a thread like this so I could create one extolling the virtues of OpenAL if one didn't already exist. It gets a lot of flak, especially from developers trying to justify their use of inferior, but easier to implement, solutions like FMOD and Xaudio2, which are abominable in terms of positioning and realism but highly portable.
OpenAL Soft was updated last year to implement HRTF (head-related transfer function) support. This results in headphones giving the wearer the impression that sounds are coming from in front of, behind, above, and under the user (and of course to the sides). It works by simulating two microphones placed in the ears with all the dampening of sounds based on the shape of the ears and head that that would entail. Basically, it emulates how people actually hear and how they process sounds from all directions with only two eardrums. You can find HRTF demos on YouTube and they will work with stereo headphones of any quality. I recommend listening to this one with headphones (and closed eyes):http://www.youtube.com/watch?v=IUDTlvagjJA
I used OpenAL Soft for headphones and for discrete 7.1 surround until I finally purchased Rapture3D. It features HRTF (and other things) as well as "tinted reconstruction" which attempts to adapt HRTF to stereo, quadraphonic, 5.1, 7.1, etc. speaker setups to simulate some of the improved directional perceptions of HRTF (such as sounds coming from above and below the listener). It also comes with more HRTF presets than OpenAL Soft, which can be important as certain HRTF datasets will mesh better with certain people for the simple reason that we all have different ear and head shapes.
Some games that have extraordinarily impressive positional audio via OpenAL that almost no one knows about (and none of these require a physical soundcard as I use HDMI or an onboard Realtek chip depending on what PC I'm on):
System Shock 2
S.T.A.L.K.E.R. Series (Clear Sky and Call of Pripyat are especially impressive)
Unreal Tournament 3
And of course FreeSpace Open
There are more. These are the ones I've most recently enjoyed on both Windows 7 and 8.1.
Please don't dismiss OpenAL just because Creative bought and then attempted to bury it just like they did Aureal3D. Unlike A3D, OpenAL is still under development, most notably by the OpenAL Soft developer(s) and the Rapture3D developer. Most developers spend almost no time working on their engine's audio, but almost any game with OpenAL support has serious potential that can be unleashed using the right methods. FMOD/wwise/XAudio2 may still be the future because developers find them so easy to work with and port, but it's a bleak one. Anyone with a good reference point for positional audio who has played any triple AAA titles in the last 5+ years knows this :/