Well, this only works with outlook email at the moment, but here is the claim:
…information will become much easier to find. When a new email arrives, the sender’s full communication history appears in the Xobni sidebar, including past conversations, attachments and contact details. Xobni also includes a blazing fast email search tool..
I first read about this free software in june’s issue of wired. . Sounds great, hmm? Basically, it helps you manage your inbox — but only in outlook (at least at the moment).