Football.com
http://www.football.com/
Plenty of current statistics, including Fantasy Football as well.
Football-Reference.com
http://www.pro-football-reference.com/
NFL data and statistics. Includes: "Player pages containing career statistics for almost every QB, RB, WR, or TE who played anytime after 1984; For more recent players, there is a game-by-game log of their stats, going back to 1995; Standings for each season back to 1970; For each current franchise, a page listing its records since 1970 and individual pages for each season."
National Football League
http://www.nfl.com/
This is the official NFL web site, with full news and statistics. Browsable by topic.
United States Football League
http://www.oursportscentral.com/usfl/
"Welcome to the online home of the United States Football League. The ultimate league from the 1980s, the USFL provided fans across the country with professional football in the spring and summer... On this site you can check out some of the most exciting players in the league, coaches, teams and even team uniforms. To find out more about the league, read the USFL's history, including standings and game results and a recap of their antitrust suit against the NFL. You can also view videos of some of your favorite players in the USFL multimedia archive. The Collectors' Corner features several USFL collections, and we even have a page of items for sale from the USFL and WFL."