Zotero is a free reference management tool developed as a project developed at Carnegie Mellon and supported by a small team at George Mason University.
Mendeley isn't open source like Zotero and doesn't have well-built browser plug-ins, although it has a better, more modern interface. Zotero has limitations with PDFs, but Mendeley doesn't support them at all. For Qiqqa, it is a better alternative and is open source as well. …
For a brief time I tried Mendalay, since at the time Zotero didn't have a Google Docs plug in. Mendalay is not as intuitive as Zotero and if I recall correctly, was more expensive. Zotero works very well, and ultimately I found myself back in Zotero because it's easier to use …
Zotero is much less prone to glitches than Mendeley, and has much easier to use web extensions and word processor plugins. I found Zotero easy and intuitive to use.
Like Zotero, Mendeley is also available for MacOS, Windows, and Linux. It's also available for iOS and Android, whereas Zotero is not. Mendeley is not open source but is free, as well. Zotero's browser integration is superior to Mendeley's, however.