A (short) list of applications that were very useful to me at one time, but then went away or astray. The question is, could one redo these in a current and useful way?

• Dopplr: showing simple travel plans (city and dates) to facilitate serendipitous meet-ups outside your regular movements. (went away after being acquired)
• Delicious: social bookmarking (went astray by dropping/breaking-by-redoing the social functionality, then went away). Have a project on the shelf to redo this for myself, called Linqurator.
• Skype: p2p voip (went astray by dropping p2p in favor of centralised servers, after acquisition by Microsoft). See this and this posting asking questions about the current p2p voip space.