Is there any application to keep an inventory of owned sets and pieces? I'm looking for a desktop application that keeps track of sets and pieces owned. Ideally something that would look up LEGO databases such as Peeron or others in order to add the pieces in a set automatically once on is added. Similarly, said app could tell you which sets you could build with your current pieces, much like Rebrickable.com, as mentioned in a previous question.
I know of no desktop applications that have functionality like Rebrickable, but an API has recently been released which would enable any applications to integrate with the site and display this kind of information. I know of one such application currently being developed but not ready for release yet.

disclaimer: I created Rebrickable.