I own a special guitar: the grunge guitar. One of the stings is removed (the fourth string) and all the strings are tuned down. I was writing down riffs for this guitar this morning and that got me thinking I'd share the grunge guitar idea with others. Playing on a guitar that is strangely tuned and has fewer strings makes you think about guitar playing in new ways. I thought of things I never would have thought of on a conventional guitar. I recommend using a cheap guitar for this. In my case, I use a harmony brand guitar I got from a pawn shop for aprox. 35 dollars.
Try it out!