FAB Defense, an Israeli company, makes a picatinny rail mounted glass breaker for the Glock.
A pretty interesting device. It appears to need to be cocked first and then triggered when it hits the glass, unlike emergency window breakers which come cocked from the factory and need a significant amount of force, usually 12 lbs, applied to release the spring.
It is selling for $124 at this website which ships internationally.
Hat Tip: Blue Gun Blog and GunPundit
UPDATE: Manufacturer name and website provided by DoubleTapper. Thanks DT.