I don't know about the order, but my top 5 would be...
Google/Gmail (always open in a tab)
VGChartz (almost always open in a tab)
Youtube (Soundtracks I haven't yet bought)
Wikipedia (I look up one thing, and end up going through many, many pages)
Yahoo (Spam account, but I have a lot of people who still refuse to send stuff to my gmail account. It's rather annoying.)
VGChartz is probably second behind Google and Gmail. I use my Gmail account for school and stuff like that, so there's almost always something I need to be doing on there. I typically have VGC open in another tab in case I get the urge to look something up or just browse the forums. I used to use IGN and Gametrailers for game info, but I prefer the database here. I only go to other game sites for reviews anymore.