Please add to the list in the comments (I'm sure you will anyway!)

SteepAndCheap.com (Backcountry.com's liquidation site - they also have a browser plugin to notify you of flash sales)
SierraTradingPost.com
TravelCounty.com
MyTrail.com (formerly Go Lite's gear)
REI.com Garage (they are also going to start selling their returned USED GEAR online)
Amazon.com (6pm.com especially for clothing/shoes if you know you won't need to pay to return them for size issues)

Other things to do:

Find something you like and do a Google search for that particular item to find low prices.

Install the Honey extension on your browser and it will always find you the cheapest seller on Amazon for the same item. You can also select to have it monitor the price and notify you when it drops.

Install the priceblink extension on Chrome and it will tell you when an item for sale is cheaper on some other website.