Already available in 183 countries and regions (or 187 if you go by the statistics on this page), Google Map Maker is now available in the United States. Map Maker was launched in 2008 to develop data in areas where detailed geographic information was lacking. According to Google’s blog post:
Using Map Maker, people have built out and edited the maps for 183 countries and regions around the world, and now, due to the contributions of citizen cartographers, 30 percent of people have detailed online maps of the places they live.
(Of course, there are other crowdsourced projects such as OpenStreetMap that have also created substantial amounts of geographic data worldwide.)
Edits to Map Maker are moderated but approved edits will appear in Google Maps within minutes (although the post doesn’t say how long it takes for edits to be approved). Cnet has a more detailed explanation of the user editing approval process: Google: Map your own neighborhood. Google has lets you see what edits are happening on Map Maker here.
Map Maker continues to be rolled out to needed regions of the world. You can sign up to receive an alert when a region you are interested in editing becomes available.