Right now the winner is: http://www.ereaderiq.com/. Live outside the US? Don't worry, just use the drop-down box to choose your region.
The site scores very high for ease of use, ability to see covers, read blurbs, sort via filter, and click thru to Amazon for easy downloading. And if your favorite book isn't free just yet, you can set it up to be notified when the price drops.
Now, all I need to do is find the time to read.