I'm looking for an AddOn that basically gives me direct access to WoWHead (Or another site of the sort) from the WoW client itself. Is there any such thing?

AddOns can't access the Internet.  The closest you'll find is LightHeaded, which makes WowHead's quest database accessible from inside WoW, by copying it to your computer.  The author posts it on wowinterface.com, since that's run by the same people who run WowHead.

No, but there are two ways of doing this:

playing in windowed mode and having your browser open

or downloading the Rogue browser from gotgame.com and overlaying it by pressing the keyboard shortcut