Darwin's World is a d20 Modern post-apocalyptic game, by Dominic Covey. Versions for Savage Worlds and True20 game systems were created too.
Unlike many existing post-apocalyptic role-playing games, Darwin's World is often described as a darker and more "realistic" game system. In specific, its use of real-life deformities and genetic diseases to portray character defects, as well as issues like Slavery, racism, and drug use give it a grittier quality than most post-apocalyptic RPGs, which often have a fantastic or "comic book" feel that requires a broader willingness to suspend disbelief
While the original Darwin's World books clearly put forth a timeline and history, the second edition (and most recent edition, v2.5) has a broader focus intending to provide rules, suggestions, and guidelines for any kind of post-apocalyptic campaign.
The default setting remains a wasteland created by global Nuclear/Chemical/Biological Warfare, however. There is a strong focus on regions that once constituted the Midwest and Western US; numerous Modules published for the game are nominally set in "desert" or "wasteland" areas. More specific locations include the desert trade town of Tucumari, the sunken city of Bakersfield and its strange subterranean monsters, the overgrown Ruins of Los Angeles, a San Fransico overrun by anti-technology zealots, and a decadent society thriving in the futuristic Domed Cities of the Midwest. One supplement went into great detail exploring the fate of the Pacific Northwest