The Super Joybox 5 Pro is the adapter many of us have been waiting for. Four players can play at the same time, it is a HID device and therefore doesn't need drivers.

4 player adapter compatible with PS and PS2 joypads
Use your PS/PS2 Joypad, Racing wheel or Dance mat with your PC
Supports all buttons, including the two analogue knobs
Easy to use, plug and play, no drivers required
Supports digital and analogue mode and pressure sensitive buttons
Allows 4 players to play at the same time
Compatible with Windows 98, Windows 98SE, Windows ME, Windows 2000, Windows XP & Windows Vista
Button re-mapping and pressure sensitive button features