After using Windows operating systems for a long time (and still am), I’ve assembled my own utility toolset that got installed on every Windows machine I used. With regards to photo viewing, I was faithful to IrfanView for fast, random photo viewing and otherwise used IMatch, Lightroom and Photoshop for more advanced tasks.
On Mac OS X, there is Quicklook (just press spacebar on an image), then there is the Preview, but neither is well suited for ordinary photo viewing, where you sometimes crop an image and convert it to another format. IPhoto wants to import images and is most importantly not fast for viewing a single images now and then (or a folder of images). So, I found Xee, which seems great so far.
You should try Linux …
I actually have… but that’s another story.