In addition to the smaller Airbuses, 737s (especially those of Southwest Airlines) are a very common sight. At the very least, there are different variations.

This Westjet aircraft is a 737-800. The 737-800 is a stretched version of the 737-700, and replaces the 737-400. It also filled the gap left by the decision to discontinue the McDonnell Douglas MD-80 and MD-90 following Boeing's merger with McDonnell Douglas. The −800 was launched by Hapag-Lloyd Flug (now TUIfly) in 1994 and entered service in 1998. The 737-800 seats 162 passengers in a two-class layout, or 189 in one class, and competes with the A320. For many airlines in the U.S., the 737-800 replaced aging Boeing 727-200 trijets.