With SharedCopy, you can add text comments or yellow sticky notes on web pages, highlight existing text using a virtual yellow marker and even draw semi-transparent shapes to emphasize important areas of a webpage.
The sticky notes and shapes can be resized or you can place them anywhere on the webpage by simple drag-n-drop. The annotated webpages have their own unique URL which can either be private or you can share it with anyone on the internet.
SharedCopy is done in AJAX and is pretty fast. A fantastic tool for gathering feedback about a webpage or creating quick site tours. All annotation objects on a webpage are available as an RSS feed as well. So if the owner add new sticky notes or changes something, you are immediately notified.
Wish they include support for pointing arrows as well.
SharedCopy.com | Founder | Webpage with Annotations
SharedCopy may remind you of another service called Fleck that also adds sticky notes on webpages with the annotation tools available inside a floating toolbar.
Related: Add Comments in GeoCities or Google Pages