I use the English Wikipedia to learn about interesting topics. Since I don't have time to read all of them, whenever a new interesting topic shows up I add it to the Watchlist. Then maybe I read them when they pop up, or start reading fragments from the diffs, eventually I can help deterring a spammer, fixing a typo, checking that a link is still valid...
More in my user page on Meta-Wiki.
I enjoy digging in the meaning and etymology of words. If I would have all the time I would probably contribute to the lovely Wiktionary.
For my alter ego working at the Wikimedia Foundation, see User:Qgil-WMF.