Just recently, I updated my copy of Adobe Lightroom and have been playing around with that. To save having two (or more) copies of my photo libraries, I've exported the whole lot out of Aperture back to their master format and then reconfigured Aperture to use the images from an external directory and just keep the previews/metafiles in the Aperture library. This way, I can use the same source images in Lightroom as well, in the same way.

See the 'Photo Galleries' page for more info/links/examples...