Give users ability to favorite or 'like' content

I have a few different custom content types and would like to give my users the ability to favorite or ‘like’ (like Facebook, but not for Facebook) pieces of content so that they can later view a list of their favorited content.

I’ve searched everywhere but haven’t come up with a clean solution. I’m pretty comfortable with SQL and PHP but want to make sure I’m doing this in the most efficient, clean way possible.

Any ideas?