Those looking for a simple but effective bike lock should consider the Surelock Flex Key 12.  Giant's cable lock is surprisingly tough for this price point with a 12mm heavy flex cable.  It's also light at just 500g.  If you have no room in your pack or pocket from the lock you can also fix it to the bike using the quick release mounting bracket.Included in the pack are 5 keys, 4 standard and 1 LED key.  The LED key is great for nighttime commutes and dark rides.Weight - 500g (includes keys & fittings)Featues:12mm Heavy Duty flexible cableLock: X2p Double bolt locking mechanismM-Key cylinder4 standard keys and 1 LED keyQuick release mounting bracket