Derrick Story talked very highly of AfterShot Pro in his lastest podcast you can hear here: http://thedigitalstory.com/2012/01/lightroom_4_aperture.html
It's towards the end. It runs on PC's, Macs and Linux machines. Sounds promising as it has layers
which neither Aperture or LightRoom has and pulls in plugins like they are part of the program. You don't have to do round tripping and creating extraneous TIFF files. Does not have a way of adding metadata at import yet. A deal breaker for me but I like having another player in the mix.
If you are looking at Lightroom and/or Aperture, I would give the 30 day trial of AfterShot Pro a "shot", too. Derrick said it is available for $79.99. There is an ad on the product page on the podcast page stating that.