Mountain Gear has a great selection <www.mgear.com>, and although they're not really a discount shop, they have randomly discounted pieces as well as massive blowout sales. Helps to check that one frequently, and also get on their mailing list. I just got an OR Neoplume and a Mithril for less than $200 combined.
Outdoor Gear Exchange <www.gearx.com> is a 2nds, closeout, overstock resale shop like Sierra; great deals, not as good selection as Sierra.
Geartrade <www.geartrade.com> has a crappy interface and also frequent poor-quality posts, but given that it's basically a permanent online garage sale for gear, you can find some great deals here.
Those are the ones that pop into my head immediately, I may add more when I get back to the forum later in the week. With any luck, this will get other people to list their favorite places too . . . .