+1 for SportsMansGuide.com. Very good site.
I'd also like to add LuckyGunner.com to the GOOD list. Prices range from favorable to downright bad, depending on the product, but stock is updated several times a day, and they only sell ammo they have in stock (even telling you how much they have available) and can ship out that day (before 3PM) or the next business day. The key here is to check the site multiple times per day because when they do get their better priced ammo in, it gets bought up very quickly. Also, as a side note, it seems to me that they are in some way affiliated with, or at least sell from the same stock as, BulkAmmo.com.
Last Tuesday I placed an order with LuckyGunner sometime around midnight, and I had my ammo on my porch before noon on Thursday, which was a day earlier than they had quoted. I will continue to use them and SportsMansGuide as much as I can.