ImDoub. See notes here. One of the complicating things is that you really need the ML (maximum likelihood) double, not the Ln-norm lowest error double.
Network of Trust. See many previous writings here. Basically this is just collaborative filtering, but where I'm in control of my own graph, more like my "friends network" on facebook or something. This is only really awesome if it's global, which is connected to :
Unified blog / recommender accounts. The balkanization of Web 2.0 is severely reducing how useful it could be. All the seperate accounts and friend connection networks means that your return on effort is linear, instead of exponential like it should be. (an open friend system is exponential because making a new connection linearly increases your rate of new connections). This is mainly a political problem, but if you built a really good framework for it, maybe you could convince more of the little forums to drink the koolaid. Obviously the sites are opposed to this because their proprietary user-created data is their treasure.
My text -> blogger app so I can write my blog like this but have it update to blogger or whatever.