ListMixer is a meta-social bookmark service. You post bookmarks to it and then can easily forward it on to other services like del.icio.us (clean ui, very popular) or Furl (caches bookmarked pages). Another feature of ListMixer is timed existence for bookmarks. If you don't revisit the link within a set amount of time, it gets deleted.
A nice combination of some simple, but potentially quite effective, ideas courtesy of Sid Steward.
Via Marshall Kirkpatrick at The Social Software Blog