An open source (licensed under the GPL) program for Windows NT4/2000/XP which provides emulated hardware to programs running inside the virtual DOS box. This currently includes various types of sound hardware (SoundBlaster card, MPU-401 MIDI interface, AdLib), a gameport interface (for joysticks and the such), low-level CD-ROM access (for copy protection purposes) and (currently experimental) VESA support.
It's ideal for running all those old games which you desperately want to play under your modern Windows OS but which you really, really want (or need) sound/joystick/whatever support for. It's even plugin-based, so if you need to emulate some obscure piece of old hardware, you can write a virtual driver! The VDM core provides such things as low-level DMA, interrupts, memory management and I/O port access intercepting.
Note that this is all fully emulated: you don't need an actual audio card or actual gameport interface, ie it isn't just a hacky bridge to the real hardware as some manufacturers provide. It's fully configurable, so you can change the DMA channels, I/O ports and/or IRQ at which the hardware appears, as well as toggle individual plugins on and off.
Grab it from http://sourceforge.net/projects/vdmsound/, install it, and then either simply right-click the program you want to run and click 'Run in VDMS', or run the dosdrv program from an existing DOS prompt. It works damn well, although you often need to fiddle with the configuration to get it to run well. If you just *can't* get it working, there is a commercial product which does much the same thing (SoundFX) as well as various x86 emulators (such as Bochs and VMWare) which might help.