AnnotatorJS is an open source JS toolkit created by Nick Stenning while he was working for the Open Knowledge Foundation. The annotations created by AnnotatorJS are often stored at annotateit.org, a service provided by the Open Knowledge Foundation.
Hypothesis is an open source toolkit (JS client, Python server). The client, originally based on AnnotatorJS, has evolved and, over time, reduced its dependence on that toolkit. Hypothesis annotations are usually stored in the Hypothesis service. Some users have deployed independent Hypothesis servers, and at least one uses an API-compatible reimplementation of the Hypothesis server.