Another friend in St. Louis discovered an extremely rare wild grass in his backyard. Its seed apparently had been brought there from dirt he'd trucked in to fill a low spot in his lawn. There are so many tiny, poorly known insects, mites, and the like that if you become an expert in them, you'll certainly encounter strange and rarely noticed species, and possibly even species unknown to science!
The main way that most of us learn how rare or common the plants and animals we see are is by reading what our field guides say about them. Roger Conant's A Field Guide to Reptiles and Amphibians describes the Stinkpot turtle as "extraordinarily abundant," but the Yellow Mud Turtle as merely "common," while the Bog Turtle is "rare or completely absent in many regions where it once was fairly abundant."
As a general rule, the closer one is to a species' distribution limit, the rarer it becomes. The range map for the Spotted Skunk, in William Burt's A Field Guide to the Mammals, shows that the mainly southern species reaches into southern Indiana. It can be guessed that any sighting of Spotted Skunks in Evansville, Indiana, near the map's "dotted line," is likely to be noteworthy.
Also, we mustn't forget that a very few species may have altered their degree of abundance since our field guides were published. One of the most striking examples of this is the House Finch. In my beat-up old bird-field-guide, published in 1966, the House Finch range map shows the species as being completely absent from all of eastern North America except around New York City. Nowadays, however, House Finches are among the most visible visitors at bird feeders through most of eastern North America.
In my old field guide for birds, the Pileated Woodpecker is said to be "uncommon and local; a wary bird of extensive deciduous and mixed forests." In contrast, the three-volume National Audubon Society Master Guide to Birding published later in 1983, says that Pileated Woodpeckers are "found in most forested areas," and adds that "...this species seems to have adapted to life in some suburban areas." In fact, nowadays they are often spotted in city parks with at least a small wooded area. Amazingly, this spectacular woodpecker has changed its behavior so that it is no longer so wary or so rare. Whitetail Deer, coyotes, opossums, raccoons, and armadillos fall into the same category.
"Rare species" are not always "endangered species." Nonetheless, you might be interested in EE Link's Endangered Species Home Page, which has some good links to sites on rare and/or endangered species.