Zotero is a free, open source tool for saving and organizing references and creating citations and bibliographies.
The website is optimized to allow you to access and edit your account on your device, and the Zotero browser extension allows you to save items to your account.
PaperShip is a free app that gives you access to your Mendeley or Zotero library on-the-go. Available for iOS.
An optional in-app purchase allows you to annotate the articles you save.
Mendeley Web allows you to access and organize a library of sources without the need to install any software.
The Web Importer browser extension allows you to add papers to your library from any major browser.