Most of my contributions to Wikipedia have been in entries for comic books. Arguably my most lasting contribution was to split entries for comic book characters into separate sections on their publication history, and their fictional biographies; prior to that many character entries mixed these two into a single section. I think I first split these elements in the entry for Hawkman, whose history has become so convoluted since Crisis on Infinite Earths that the two elements couldn't be mixed into a single section and that section still be comprehensible. I've noticed that many other articles have adopted this structure since then, so I like to think I've made a significant contribution in this (admittedly geeky) space. :-)
I largely stopped contributed to Wikipedia after a frustrating edit war in 2004, but I may start making some minor edits again in the future.