Protonotes is a web tool that allows people to build conversation or add notes around a web page.

Protonotes allows you and your team members to add notes to prototypes in real time. When you register with Protonotes they will send a link for a page, on that page you can enter the URL you want to add as a prototype and start adding notes to it. Only one member in your team has to register to use protonotes, you can share your URL with other members and all of you can view and add notes to it.

Protonotes is a nice tool but it may only appeal to people who work with prototypes and it wont work with Firefox, you need to use IE for this one.