This infographic from the 2012 Census on Local Government shows the breakdown of districts across the United States by type:
The Environment and Housing category is pretty broad, including things like Soil and Water Conservation Districts, Levee Districts, Water Districts, Irrigation Districts, Housing Districts, and more. No individual kind of special district makes up a very large percentage of the total. These entities handle a large array of specific activities — there are 3,248 drainage and flood control districts, for example, and 3,438 housing and community development districts.
The largest group of single districts is Fire.