My sansamp GT2!!!
Man, I used to be a gear freak, and since I own that little box, I can dial any sound (pretty much) without having the army trying to bring down my house because of the volume!! I know the line 6 products do a pretty cool thing too, but this one is totally basic, three knobs, and rock on!!

Also, a good guitar strap, might sound stupid, but it's totally useful when you play live, and you keep on checking your straps so your guitar doesn't fall!!