I have assembled a partial list of websites that have been useful in developing my playing. Some of them are the websites of people I have personally worked with, either as colleagues or teachers. Others are miscellaneous resources that can be helpful to eager bassists and musicians.
Barnabás Rácz - Hungarian double bass restorer, and maker of my bass
Pirastro Strings - German string company of exceptional quality
XOTiC Guitars - modern electric bass makers - I own the XJ-1T model
JD Hill - Bass dealer in New Jersey, where I purchased my bass
Bill Merchant - excellent NYC bass repair and setup
David Gage - NYC string super-store, specializing in bass sales and repairs
Dennis Masuzzo - my former teacher, and advocate for 5ths tuning
Bulletproof Musician - psychological training for musicians
Talkbass - the ultimate bass players forum, with every possible topic discussed
Double Bass Blog - extremely useful resources for bassists, especially interviews and recordings
Lemur Music - one-stop shopping for all of your bass-playing needs - bassist's Amazon!