[izym]

Mendeley. Not the nicest UX, but it syncs, has tags relevant for academic papers and can export everything as a bibtex file.

[rmm]

I second Mendeley. Highlighting features, sharing. Awesome software

I only wish it had pen support (surface). If I could markup papers as I go with my pen it would be the ultimate tool.

[therobot24]

try qiqqa