Looking for a ThumbsPlus equivalent myself, I found this thread through Google, and find it odd no answer was ever given...
Personally I need the duplicate image finder in ThumbsPlus (on the PC, it's basically impossible to find any other that comes even close to how well the ThumbsPlus dupe finder works), and the swift ability to move/copy huge amounts of image files from one folder to another, without breaking reasonable organization. I keep pictures, video, and related text documents together by subject. 40+ GB of files, well more than 500,000 pictures alone, not counting video and other related files.
I don't care about the editing features, Photoshop and Corel Photo-Paint have always been just fine for that. But the management abilities I do need.
iPhoto isn't geared for the task, Picasa is an insult (when Windows Explorer is easier to work with than a program that's supposedly designed specifically for the task, something is wrong).
Read several recommendations for Lightroom (darkroom?) and Aperture, but the price is scary. ThumbsPlus is $60, Aperture is $200.... Aperture is a great program, but the price level is absurd for what it does, and what it can't do...