Is there a special add-on or an application, possibly web based, that I could use to co-browse websites with my friend? So that I could see the contents of my friends screen, when he's surfing the web.

Sounds like you're looking for virtual desktop software. The basic idea is that one computer runs a server, and other computers can connect to it and view (and manipulate) the desktop and windows as though they were sitting in front of the computer. There are a lot of programs out there to do that sort of thing; I use UltraVNC and it works quite nicely.

Well, at first I would have recommended VNC and other similar remote control software, but if it's just the browser screen sharing you want, then why don't you try www.***.com. It's simple and web-based.

Yea... VNC is quite a good software... I use it for all the computers at my house if I want to assist my house mates with anything...