July 14, 2019. The service has now been fully shut down.
March 11, 2017. AnnotateIt is now closed, but the project to annotate the web continues. The W3C has recently standardised web annotation, and development over at Hypothesis continues apace.
If you're interested in making your own annotations on the web, we encourage you to check out Hypothesis, which is built and maintained by the people who made Annotator and AnnotateIt.