[Osmium]

I love Papers but they've been ridiculously slow with retina support; I wouldn't mention it, except it's the easy things that still haven't been done, such as coloured circles for tags, while more complicated things (many icons/artwork etc.) were done ages ago.

They released a major new version (Papers 3) just under a year ago. One of those 'fresh start' kinda releases; a re-think of what a reference app should be like, and substantially different from Papers 2. I still haven't upgraded because it doesn't do much more than Papers 2 did, but if you're coming to the product for the first time, it really is a great release. Highly recommended. Definitely the best Mac client out there, and they have a good iOS app too, along with solid Dropbox syncing. (Haven't tried their Windows version.)