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.

i think kayako has a feature like that or you could use a vnc software like realvnc

Kayako's ViewShare feature would share staffs screen with the client. You can look into RealVNC, LogMeIn or GoToMeeting as well.

I'd recommend LogMeIn if you are looking for the ability to have both people interact with the desktop at the same time.

If you and your friend have Macs, running Mac OS X Leopard, iChat has a screen sharing feature built in.

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 Screen2.com. It's simple and web-based.