Here's a list from the M2s at my school. Just started our physio/histo class today so I haven't looked at any of these sites yet. Our profs suggested the Iowa virtual slidebox, but that might be biased more than a little bit.
The virtual slide box is the best. It features real-time zooming features and mouse-enabled drag and scroll. It's also great for using the pictures to make PowerPoint slides to quiz your peers.