Guessing at agency acronyms isn't helpful if you're trying to track them down. Here are some starters. As has been mentioned above, the requirements for most of these jobs may say you need a high school diploma, but when so many who apply have at least a 4-year degree, chances are slim that they'll get past that lot to start picking up non-college folks unless they have a lot of extra points from military service and other protected categories of workers.

I've broken down the agencies and given a little information about some of them here. For all sorts of information these days you can start your search at http://www.firstgov.gov/.

USDA--United States Department of Agriculture

Forest Service--primary USDA agency with outdoors jobs, primarily foresters, if you're looking for timber management work or fire fighters if you're looking for hard work, excitement and lots of boredom commingled. There are interpretive jobs in the forest service and some recreation jobs. I think they're still classed as forestry aid and tech, not park aid and tech.

USDI--United States Department of the Interior

National Park Service--the biggest agency with "ranger" jobs, also an agency with the attitude that "if you don't like the conditions, quit--there are plenty of people in line who want to replace you." Park aid and tech. Other jobs have different descriptions, and can be a foot in the door. I knew several friends who started in maintenance jobs then moved over into the park ranger jobs once they had a foot in the door. They have some firefighting here also, but not as much as USFS.

Bureau of Land Management (BLM)--they have some very nice ranger jobs. I once turned down the job of river ranger on the Missouri, because they rated my application wrong and offered wages lower than I was willing to accept. Darn. They came back with another offer, but I'd already taken a different job. Lots of firefighting here.

Bureau of Reclamation (BOR)--they have seasonal jobs, but I don't know if they have rangers these days.

Bureau of Indian Affairs (BIA)--a dicey agency to get involved with. If you're Indian, give it a try. There's lots of infighting and some pretty loose management of this agency. The USDI is still clueless as to how to fix things here.

U.S. Fish and Wildlife Service (USFWS)--there are some very nice seasonal jobs with these folks.

United States Geological Survey (USGS)--don't be surprised--someone has to do the on-the-ground work with all of those maps. I don't know if they have seasonals now, but I think they used to.

United States Army

Corps of Engineers (Civilian) (COE)
Lots of park rangers on their various dam projects and waterways. They're more like a dumbed-down NPS, few interpretive jobs, mostly just crowd control, law enforcement, and inspecting the work of contractors. In the summer when the campgrounds are filled with visitors there used to be limited interpretive programs.

SRS