Photo browsing and viewing application
I use Picasa from Google for that.

It is not intrusive (will leave your photos in place), and it will not make duplicates (saves space on your disk).

It also comes with Google web space (so you can upload/download) and easy share with others.

Here is what they say about them self:


  
  Picasa does not store the photos on your computer.
  
  
  When you open Picasa, it simply looks at the folders on your computer
  and displays the photos it finds. It displays the file types that you
  tell it to find, in the folders that you tell it to search.
  
  
  Your original photos are always preserved.
  
  
  When using editing tools in Picasa, your original files are never
  touched. The photo edits you make are only viewable in Picasa until
  you decide to save your changes. Even then, Picasa creates a new
  version of the photo with your edits applied, leaving the original
  file totally preserved.