If you've got a onePointFive (1.5") headtube bike, but would prefer to use standard 1 1/8" stems and forks, the Double Xc Reducing Headset from Cane Creek is the answer. Like it's big brother, the Double X, it offers a 20mm cup insertion depth, providing more strength to the headtube/cup interface and includes 18 millimeters of Interlok aluminum spacers, which provide a tighter fit with the headset/stem interface. But thanks to it's special design, you can use any 1 1/8" suspension fork.
 

The XC short works for Cannondale (except Raven) headtubes.  This is facilitated by using shorter cups.  The Double XC short only inserts 12mm into the headtube instead of 20mm like the regular Double XC.