I need a way to allow users to rate the helpfulness of documentation articles on our docs site. Usabilla is an example of a tool that is created for this (and other things). What other ways exist for collecting and tracking this kind of information?
It depends on the depth of the information you would like to collect. If you want to know what volume of users find your article useful it`s enough to use a simple javascript where users can press thumbs up or down on each article or use Usabilla or other competitors of their for more indepth data on users.