MidwayUSA is more or less a well-organized online Wal-Mart of shooter supplies, in contrast with Cheaper Than Dirt (a badly organized online Wal-Mart).
They do tend to slant their inventory towards the cheap vendors, so do a little research before choosing their items. They generally don't stock junk, but more towards the middle of the road. On the other hand, many things (i.e. case tumblers, ammo boxes, etc) don't require premium products in order to perform satisfactorily. They do honor Curio+Relic discounts, and are overall a pleasant shopping experience.
Californians should note that, unlike Cheaper Than Dirt, they will ship legal AR-15 and AK-47 supplies to California. CTD and Sportsman's Guide both have well publicized and oft-lamented policies penalizing legal Californian gun owners.
Website link: http://www.midwayusa.com/
-- SeanNewton - 31 Oct 2006