12mm triple square socket with a 3/8" drive, also often called a 12 point or xzn socket.

Applications where it is needed:
 - Mk2 and Mk3:
   - head bolts
 - Mk4:
   - removing the axles flanges from the transmission
   - head bolts(verify size as some use a 10mm)
 - Mk5 and Mk6:
   - removing the front wheel bearing/hub