Most important features: - Easily show contents of multiple nested folders at once (flatten folder structure) - Support for all important file types, 32 Bit images. I used Picasa: Some files not supported like 32bit, Works better because you can select expand all for folders. I used ACDSee: Most supported file types, but…
I'm still on CS6 and it's pretty decent. I don't use it that often but I've had a folder structure open with over 3000 images inside and it loads quite fast. Plus it has a handy filter feature for sub-folders.
@EliasWick guessed correct. :) Bridge does indeed fit your needs. It displays all the usual files: psd/tif/tga/png.hdr/exr/etc And you can show a 'flat view' of all sub-folders. To do this click View>check Show Items From Subfolders>Right click the folder you want>choose Show Items From Subfolders.
Have you tried digikam? It's free/opensource and has flat folder viewing with subfolders afaik. I can't find any information on whether it supports 32 bpp images but given that it costs nothing shouldn't be hard for you to test.