Marginotes takes your jQuery selection and adds notes to the margin with the text provided in HTML attributes.
jNottery is a jQuery plugin that lets you add notes and markers to webpages. All the data is encoded as a part of an URL which makes it easy to share or save.
noty is a jQuery plugin that makes it easy to create alert, success, error and confirmation messages as an alternative the standard alert dialog. The notifications can be positioned at the top, topCenter (like GMail), bottom, centre, top left or top right. There are lots of other options in the API to customise the text, animation, speed, buttons and much more.