What law enforcement agencies use Glock handguns?

There are several. Offhand, I know the Johnston County Sheriff's Office in Johnston County, North Carolina issues the Glock 23 as its standard sidearm. There are bound to be countless other agencies.

If I recall correctly, Glock pistols are also used by Firearms Authorised Officers in the United Kingdom, and I remember seeing the police with Glocks in Mexico City.