Kindle vs. Nook
There's a pretty good head-to-head comparison here, if you're interested. Honestly, if I were buying one today I don't know which I'd go with. The Kindle wins on organization since they've now implemented a "collections" system to help organize the books, which is necessary fe to keep my library easily accessible on the device. But... The Nook wins on price (for the WiFi-only version, anyway) and because it uses the industry-standard ePub file format (which helps avoid platform lock-in).
If you're thinking of picking up an e-reader (the cheap Nook is only $150 now) then you really ought to read the comparison.