I use Mendeley, and have found to be quite good for my needs. Previously I used ReadCube, which was also quite good - unfortunately it doesn't run on Linux, so

I use Mendeley, and have found to be quite good for my needs. Previously I used ReadCube, which was also quite good - unfortunately it doesn't run on Linux, so I had to ditch it. I would love to try out Citavi, it sounds even better than Mendeley, but again it doesn't do Linux. (Note: Citavi carries a fairly hefty price tag, but if you're affiliated with some university, chances are you can get it for free. The other two are freeware with the option to upgrade to premium plans.) All of these are closed-source, so if you're an FSF-fan, steer well clear. Otherwise: well worth the use of!