Harry Harrison - stainless steel rat of course, also great Viking series (hammer and cross), desthworld, to the stars trilogy.
Anything by Philip Jose Farmer - lots of rip roaring adventure stuff including some Tarzan books, Doc Caliban (his version of Doc Savage) tierworld is great, but eclipsed by Riverworld which has an amazing cast of characters including Sir Richard Burton, Alice Hargreaves, Samuel Clemens (Mark Twain), King John of England, Cyrano de Bergerac, Tom Mix, Mozart, Jack London, Lothar von Richthofen, and Hermann Göring.