Southwest Airlines serves Baltimore with nonstop flights from:
Albany, NY
Albuquerque, NM
Austin, TX
Birmingham, AL
Buffalo, NY
Chicago, IL
Cleveland, OH
Columbus, OH
Denver, CO
Detroit, MI
Ft. Lauderdale, FL
Ft. Myers, FL
Hartford, CT
Houston Hobby, TX
Indianapolis, IN
Jackson, MS
Jacksonville, FL
Kansas City, KS
Las Vegas, NV
Little Rock, AR
Long Island, NY
Louisville, KY
Manchester, NH
Nashville, TN
New Orleans, LA
Norfolk, VA
Oklahoma City, OK
Orlando, FL
Phoenix, AZ
Pittsburgh, PA
Providence, RI
Raleigh-Durham, NC
Salt Lake City, UT
San Antonio, TX
San Diego, CA

One-stop service from most other airports. Other airlines, too!