The Geography & Map Division at the Library of Congress is constantly adding digitized versions of historical maps to their online collection/database.
The G&M Division publishes New Maps This Month!, a roundup with direct links to each new addition along with basic bibliographic data.
You can access the February/March 2015 update (52 pages; PDF) here. We’re also embedding a full text copy below.
Previous issues of New Maps This Month! (back to May 2011) are also available.