Southwest Airlines has begun service at Denver International Airport with 13 daily nonstop flights to Chicago Midway, Las Vegas and Phoenix. The airline also said that beginning March 4 it will offer new nonstop service between Denver and Baltimore/Washington (one daily nonstop departure) and between Denver and Salt Lake City (four daily nonstops). Nonstop flights to Baltimore/Washington start as low as $99 one-way with 21-day advance purchase, and nonstop flights to Salt Lake City start at $49 one-way with 21-day advance purchase. Also, beginning March 4, 2006, Southwest will add a fifth daily nonstop flight between Denver and Phoenix and a sixth daily nonstop between Denver and Las Vegas. Visit [www.southwest.com].