The BASSMIDI MIDI synthesizer driver is a freeware SoundFont based Windows MIDI system driver (created by Kode54 and MudLord) for Windows XP, Windows Vista, and Windows 7. It uses the BASS and BASSMIDI audio libraries by Ian Luck (Un4seen Developments) as the SoundFont synthesizer, and includes a BASSMIDI Driver configuration utility.
The BASSMIDI SoundFont compatible MIDI synthesizer driver features:
Dynamic sample caching (so it's super efficient on RAM usage).
Unlimited polyphony.
SoundFont chaining and stacking (using a simple list file "BASSMIDI.SFList").
Global volume slider to set master BASSMIDI Driver MIDI playback volume.
Works on Windows XP, Windows Vista, and Windows 7, including 64-bit versions.
Extremely low latency (almost zero, depending on CPU speed and operating system, and BASSMIDI Driver version); perfect for real-time performance, as well as MIDI sequencing (with a MIDI sequencer such as Cakewalk Professional v3.01).
The best sounding SoundFont compatible software synthesizer, hands down... with the closest sounding MIDI playback to real E-mu SoundFont hardware available!
Support for the following MIDI controllers and events:
MIDI note events.
MIDI program events.
Master volume level.
Channel pressure.
Pitch wheel.
Bank change MSB (CC#0).
Modulation (CC#1).
Portamento (CC#65, CC#84, and CC#5).
Volume (CC#7).
Panning (CC#10).
Expression (CC#11).
Sustain pedal (CC#64).
Soft pedal (CC#67).
Low-pass filter resonance (CC#71 or NRPN 121h).
Release time (CC#72 or NRPN 166h).
Attack time (CC#73 or NRPN 163h).
Low-pass filter cut-off (CC#74 or NRPN 120h).
Reverb send level (CC#91).
Chorus send level (CC#93).
Stop all sounds (CC#120).
Reset all controllers (CC#121).
Release all keys (CC#123).
Mono/poly mode (CC#126 and CC#127, respectively).
Pitch wheel range (RPN 0).
Fine tuning (RPN 1).
Coarse tuning (RPN 2).
Support for the following MIDI events (if GS or GM2 mode is activated):
Drum key low-pass filter cut-off (NRPN 14knh).
Drum key low-pass filter resonance (NRPN 15knh).
Drum key coarse tune (NRPN 18knh).
Drum key fine tune (NRPN 19knh).
Drum key volume level (NRPN 1Aknh).
Drum key panning (NRPN 1Cknh).
Drum key reverb send level (NRPN 1Dknh).
Drum key chorus send level (NRPN 1Eknh).
Basically, this driver is a beast. It supports ALL of the features of General MIDI 2 and Yamaha XG. More Information Here
Download
You can find the complete installation guide here
EDIT: According to DarkMatt, VirtualMidiSynth may be a better choice. I haven't personally tested it, so use at your own risk.
For those of you who want to have sound thats close to the Yamaha XG soundset, you can download this soundfont . For those of you who prefer the original MIDI sounds, or want to use it for testing purposes, use this Soundfont.
If you have a powerful PC, you should use THIS SOUNDFONT.(Thank Sephiroth for hosting it.) It is massive, but it sounds REALLY REALLY nice!
Check back for AZCEMS 2, and a guide to using FL Studio to access the extra features of BASSMIDI.
Updated 1/11/2013