There are two graphic programs I use when I need to find, view, organize or convert pictures on my computer. Both are free, of course and do what they do well.
XnView - I’ve been using it for years after
ACDSee became bloated and slow. It’s the fastest, most feature-rich and capable image viewer I’ve ever had the pleasure of using and is updated constantly. You can whip through directories of graphics with the thumbnail browser standard in all programs of this type, batch rename/ convert/ edit files and create slide shows. While you can do basic editing of graphics with XnView, I prefer dedicated graphic editors like Adobe Elements or Paint .NET. XnView is light on system resources, doesn’t eat up much memory and you don’t have to install it; you can use it as a portable application. XnView supports over 400 different file types including movies and PDF files, it’s cross-platform and according to their website “there are versions for Windows, MacOS X, Linux x86, Linux ppc, FreeBSD x86, OpenBSD x86, NetBSD x86, Solaris sparc, Solaris x86, Irix mips, HP-UX and AIX!” Chances are no matter what OS you use you can run XnView! (For the record, I did try Irfanview several times over the years but I refuse to use anything that has such a horridly ancient & outdated GUI). 
Picasa - has a ton of features
and while it’s categorized as an image viewer it’s really an image organizer. I’ve seen a lot of image organizers out there and while free doesn’t always mean something’s comparable to a paid product, Picasa is both FREE and GOOD. Like XnView you can edit pictures but that’s not really it’s strong suit. Picasa can scan your whole computer looking for pictures and movies and automatically scan for new ones on your system and add them to it’s index. After the pics and movies are scanned you can whip through them all fast as lightning in a thumbnail view or search by name or tags you assign your photos for better organization. Using Picasa, it was very easy to locate pictures I thought I lost and misplaced. You can also put your pics into different album views which makes sorting your images a snap. While Picasa uses more memory than XnView that’s to be expected from a program that organizes all your images vs a picture viewer and Picasa didn’t run slow on an older backup machine I have vs. my main rig.
Picasa can also upload your pictures to Picasa Web, Google’s free web album. It also integrates well with Blogger and you can email, Geotag, export or print selected pics as well as creating movies presentations or collages.
It’s free from the folks at Google, but only works on Windows and Mac OS X (beta). Sadly, there was a Linux version but it’s no longer being devloped (boo!).
XnView and Picasa make a great combination for practically anyone who has a lot of images on their computer and best of all they’re free and feature-rich.
What programs do you use for viewing and organizing your pictures?