I recently added an index of seedless plants to the database of plants on Plants of the Northeastern U.S. This is intended to make it easier to find pages related to ferns, horsetails, clubmosses, mosses, and liverworts. There are still quite a few mosses to be added, including Bryum argenteum (below).
This species is easily identified as it grows in very short, dense colonies and has a silvery overall appearance. It is common in dry, open areas like driveways and sidewalks.