A software MIDI synthesizer
On 32bits systems some better alternatives exist, like the well-known YAMAHA XG SoftSynthetizer S-YXG50 and Cakewalk-sfz+ (both unsupported now).
VirtualMIDISynth is a software MIDI synthesizer implemented as a Windows multimedia user driver, accessible as a standard MIDI Out device. If you are a MIDI/karaoke fan, like me, you know that MIDI play quality on Windows systems was not really good out of the box. The bundled "Microsoft GS Wavetable Synth" MIDI Out device is just... ehm... simple: poor samples, mono sound, no effects (reverb, chorus) support.