Librarians live to classify and it is helpful to organize the new metrics into categories—a sort of who-to-whom matrix of interacting variables—defining measurables (saves, tweets, posts, shares, likes, views, downloads, etc.) and the sources of who or what is being assessed (single articles or other publications, individual authors, journals, or publishing platforms, or organizations such as labs, academic departments, universities, etc.). Additionally, it is useful to know which websites and tools individuals can access freely themselves, and which are available only to institutional subscribers. Unfortunately, the artificially neat lines of any organizing matrix soon begin to blur and dissolve, as some tools, such as peer networks like Mendeley or Academia, act as both source data and the measure itself, while other tools, such as Altmetric, offer limited free access to individuals and a more fully functioning product for purchase by institutions, funding agencies, or publishers. What follows is a list of the main resources organized by basic function, with occasional hopscotching across categories.