Most major cruise lines, including Royal Caribbean, Celebrity, Princess and NCL (plus some smaller cruise lines) offer a few Panama Canal sailings each year, predominantly on repositioning cruises between the Caribbean and Alaska, Westbound/Northbound in the Spring, and Eastbound/Southbound in the Fall.
Princess designed two ships specifically to sail the Panama Canal, the Panamax size ships Island Princess and Coral Princess, which sail the Panama Canal throughout the Winter season, with each ship featuring more than 700 veranda staterooms.
Smaller, expedition-type ships, such as Lindblad-National Geographic, and various tour operators known for river cruises, including Tauck and Grand Circle Travel.
A number of land tour operators service Panama and the adjacent Central America countries. If you are going for a cruise to the Panama Canal, your professional Cruise Consultant can arrange a comprehensive shore excursion, so you can see the "most" of Panama.