This article is from the Locksmithing FAQ, by Joe "Spike" Ilacqua spike@indra.com and Henry Schaffer hes@ncsu.edu, major data collection effort by Scott Anguish sanguish@digifix.com with numerous contributions by others.
Easiest: If you registered your lock, call or write Kryptonite
for a new key. Or call a local locksmith, they should be able to
pick and re-key the lock for you.
Easy: Get a car jack and jack it apart. Careful, otherwise it is
very possible that you'll damage the bike.
Easy: Use a cut-off wheel in a Dremel tool to cut the lock at
the hole in the shackle (where there is the least to cut.)
Harder: If it doesn't have the newer brass jacket, peel back
the plastic coating on the key end, drill out the pin that
holds in the cylinder, remove the cylinder, open.
Hardest: Chill the metal of the "U" with liquid Nitrogen or
Freon, smash with hammer. While this is a "well known" method,
it may be an urban legend.
 
Continue to: