These are some of the dive operators and shops that I have used and recommend.  I especially like Amigos del Mar in Los Cabos.  Kevin and Jane and the rest of the crew there run a very friendly dive operation that I have been enjoying for the past 4 years.
http://www.amigosdelmar.com/index.html

The Solmar V is a real treat if you want to see big animals.  I dove the Socorro Islands with them over new year’s 2009-2010 and had some awesome open ocean diving experiences.
http://www.solmarv.com/

If you’re in Playa del Carmen on the Mayan Riviera look up Playa Scuba with Luis and Klaus.  They took the group I was with to the Dos Ojos Cenote for some fresh water cave diving.  I felt very comfortable in a cave environment with them.
http://www.playascuba.com/english/whoweare/index.html

If you find yourself in Cozumel check out Palancar Dive located at the Occidental Grand located a bit south of town.
http://divepalancar.com/cozumel_instruction.php
Or closer to town is Scuba Club Cozumel.  They are located walking distance to town and are totally set up for divers.
http://www.scubaclubcozumel.com/

When you’re in Puerto Vallarta you can dive with Banderas Scuba Republic.  Cesar Ortega runs this dive operation and he offers very personal service.  I dove with him for 5 days in November 2009 and enjoyed some pretty nice dives.
http://www.bs-republic.com/in_index2.htm

When I was in Maui I used the services of Lahaina Divers located in Lahaina and was very satisfied with their service.  They are located at 143 Dickenson St., Lahaina.
http://www.lahainadivers.com/

The local dive shops that I recommend are Bamboo Reef in Rohnert Park 707 586 0272
Stop in and say hi to Pam, the manager, who is really knowledgeable about diving and equipment.

Pinnacles Dive Center is located in Santa Rosa 707 542 3100 and is run by Scott Taylor.
Drop by and say hi.