I use http://trakt.tv mostly because it scrobbles automatically (from XBMC and a bunch of other media centers) what i have seen instead of marking everything by hand.
Checked the website referral logs which led me to this topic :) I'm one of the founders of trakt and right now we have real time scrobbling plugins for XBMC and MediaPortal. Boxee and Plex clients are beta tested right now as well. Check out the site or hit us up @trakt if you are interested in beta testing Boxee or Plex.