I've read a lot of the posts on the site and I've found them to be interesting and informative.  I've also seen numerous posts, asking which agencies and departments don't use polygraphs as a pre-screening tool for hiring.  I have had aspirations of getting into law enforcement since forever, but mine have always been geared toward federal service.  I have a small, but helpful list for those looking to get into LE that don't want to chance their dreams on the pseudoscience known as the polygraph. the education requirements tend to be higher amongst federal agencies, but the perks and training are a bit better than your local pd or sheriffs dept.  anyhow, here it is.......
ICE (Immigration Customs Enforcement)
USDA Forest Service (LE division)
US Marshals Service
Federal Protective Services (encompasses the GS-0083 Police Officer Job Series, civilian positions within all military brances, government offices etc) 
Bureau of Indian Affairs
US Fish and Wildlife Service (Special Agents/Investigators)
National Park Service Police
National Institute of Health (Police Branch)
US Postal Service, Office of the Inspector General
Customs and Border Enforcement
Bureau of Prisons (Investigators)
Internal Revenue Service (Special Agents/Investigators)

Hope this list helps out, I'm sure I haven't found them all, but this is a start.

ICE, U.S. Marshalls, and the U.S. Postal Service (to become a postal inspector) all DO use the poly on applicants.