Peer with your LJ friends when they go online. (LiveJournal letting you know who to peer with....)
Download LJ entries, comments, userpics, photos, etc from them.... probably high hit rate: good possibility the people they're reading are your friends too.
The LJ server would then know who is logged in and not, and the P2P client would also listen to http://localhost:nnn/ and we could return userpic requests and such directly to that, which does its P2P thing.... or just redirect them to that for their friends page.
Um.... anybody want a project? No GUI's really needed... could all be web-based UI so it's portable.
(I think I've talked about this before, but I never considered the friends-P2P thing even though it's so obvious)
The question is what feature to offer to entice people to use it. Keeping LJ's bandwidth costs sane isn't incentive enough for most people. Maybe giving them a free paid account after some measurable gain by using it? (Freeing up CPU by having their client do the friend page aggregation and freeing bandwidth by having them cache things....)
Man, I'm so developing for Avalon/.NET when Microsoft pushes their fat clients. Mozilla/Mono people better book it. (.NET has a concept of "local storage" which is like cookies on crack....)