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.
There is a tool called BrickStore that might do what you want. It is tightly connected to BrickLink.
As far as a desktop app for inventorying pieces, I personally use Brickstore as suggested by @Superkalle. Although I use it primarily for preparing Bricklink orders rather than a catalogue of my spare parts and/or sets.
Should you be interested in an alternative that may be more suited to keeping track of pieces you own, there is 'Parts Catalog' from Bong 69 Productions.
From their website:
Parts Catalog is a program used to inventory your collection of Lego parts. The main goal is to be able to check 2 list to see which parts are missing. To build your list you could just download certified list of Lego sets and Combine the list. Then add any single parts that may not be from a set.
Features list (from their website):
- Make and save your own inventory list
- Combine and save inventory list
- Sort parts by Part Number, Description, Color, or Count
- View an image of the part (if an image is available)
- Compare 2 list to see which parts are missing.
- Compare 2 list to see which parts you have.
- Make an inventory list from Ldraw dat files.
- Open/Save a list in the Peeron (.prn) format
- Open/Save a list in the CSV (.csv) format
- Open/Save a list in the XLS (.xls) format
- Open a list in the Bricklink XML (.xml) format
- Open/Save a list in the Bricklink (.blk) format
- Open a .htm inventory file saved from the peeron site.
- Render images using LDraw dat files
- Zoom and Rotate images that are rendered with LDraw dat files
- Download inventories from peeron
- Make labels from LDraw files