Today I recommend a tool for managing your papers that store in you mind. Mendeley, an all-platform tool for managing your references.

• It is web-based, and provide apps for Mac OSX, Linux, Windows and iOS and syncs your library across them.

• It’s free for personal use with a limit of 2GB, which should be enough for most users.

  Pay if you need more storage or group cooperations.

• Another feature I love most is that it can analysis pdf files automatically.

• Of course, you can highlight and annotate interesting sections and record your thoughts, and they are part of your library and will syncs across all your devices.
• It is kind of academic searching engine itself. You can discover more or search by your knowledge.

I won’t say Mendeley is the best one, but it’s much better than keeping everything in mind and downloading some paper every time you want to read it again:)