As a web enthusiast, I've registered with so many sites that I lost account of the number of profiles I maintain (or ought to!). That said, below is my best shot at listing at least the main ones:
About.me Academia.edu CouchSurfing Del.icio.us DeviantArt Disqus Facebook Flickr Flixster FriendFeed Geni Google Google Reader Identi.ca Last.fm LinkedIn OpenStreetMap Quora Twitter UserScripts UserStyles Wikimedia Yahoo Answers Youtube
p.s. - If you're curious, take a look at the source code of this page to see the full list.