Mendeley has a both a desktop client and a web version. Using both together works really well - in the web version, you can easily add citations from any computer, and there is a "suggest" function that will show you similar articles to the one in your Mendeley library. The desktop version is great for editing citations, adding tags, and adding references to your paper-in-progress.
You can add citations to your paper in progress with a Microsoft Word plugin under the "Tools" menu.
If you use another word-processing application, you can just drag and drop the citation from Mendeley into your Google doc or Pages file for the bibliography. You'll have to manually enter the author-date for in-text citations.
Mendeley Web vs. Mendeley Desktop, from: http://libguides.rollins.edu/mendeley/webvsdesktop
Mendeley has a full list of help guides (Desktop, Web, iOS, Android, etc) as well as a full series of tutorials here: https://community.mendeley.com/guides