This small microcontroller-based circuit will allow the use of a Namco neGcon PSX controller with the PS3, by emulating a SIXAXIS controller. Although originally intended for use with Wipeout HD, it should work with any racing game, although it will not necessarily provide perfect analog control in all online games due to lag (see here for details).
Download the files here.
For more information, please visit the thread that started it off: PS3 Negcon - Can it be done? at WipeoutZone.com.