I'm looking to start a new build soon. This case is speaking to me.
http://www.tigerdirect.com/applications/SearchTools/item-det...
I used it for my last build. You can select a variety of different vendors, browse parts compare prices, etc. It does a pretty good job doing the compatibility checks for you as well (I can do them myself, but my heart just wasn't in it this time, so it was handy).
I basically assembled my build with pcpartpicker, selected newegg, amazon and microcenter, then bundled it all up and ordered it. The microcenter one was neat because they actually found the processor I wanted at a local microcenter, for in store pickup, at a hundred bucks cheaper than anywhere else (not a technically crazy hurdle, but really convenient and pleasantly surprising).
Wondering how it compares to the site linked (I haven't looked through it yet; they could be totally different, but I'm going to check it out in a sec)
Edit: Also, the first several builds I made to test the site out all appear to be shipping from Amazon. Not sure if this supports multiple vendors or not.
It'll let you compare the price of a part on newegg with the current price on amazon and tigerdirect.
When I rebuilt my htpc about 6 months ago, I was surprised to find that many of the parts were actually cheaper on amazon.