I use CDisplay.I've tried most of the alternatives, but I'm just a lazy bum and it's simplicity keeps me enslaved.
I love the way CDisplay 'catches' at the end of each page when you scroll with the mousewheel, so you need five extra 'wheel clicks' to get to the next page. I also like that it uses the entire screen (no borders).
I don't like the way it scrolls double-pages. I find myself backing up and re-scrolling the page repeatedly to see the whole thing. I'm not sure of a better way to handle it though, since reducing the size makes them unreadable to me.