@dfoster85 - Oh, there are too many to count almost! Within the TV/movie realm, you've probably seen the Drug Enforcement Agency (DEA), US Marshal Service (part of the Department of Justice), and the NCIS (Navy Criminal Investigative Service). The other branches of the military have their own investigative units, too.
The Department of Homeland Security has several different agencies, too--Border Patrol, immigration, etc.
Then you have the Park Police and a whole bunch of others, right down to the Bureau of Engraving and Printing Police (who knew there was such a thing?).