Annopad is a free extension, described as a notepad/bookmark hybrid, for Chrome and Firefox that allows you to take notes while on any website. You can create, save, and export URL specific notes to save notes, reminders and more.
Click on the extension while on a website and add notes about that site. It saves the URL of the site with the note. It's very easy to use and can be a great tool for doing research and for students to take notes while on a site.
Other note taking tools like Evernote and OneNote capture the URL when you clip a webpage and you can add notes to that clipping later, but this is much simpler, quicker and easier to use while still on the web page.