Sleazy Fork is available in English.
Can work on almost any webcomic/manga page, preloads 5 or more pages ahead (or behind), navigates via ajax for instant-page-change, lets you use the keyboard, remembers your progress, and it's relatively easy to add new sites
oglaf.com no longer working
Firefox developer console says
Error: set[0]: xpath: //img[starts-with(@src,"http://media.oglaf.com/comic/")]