I'm amazed by Picasa Photo Viewer behavior! The way this thing works simply changes my picture viewing experience. It is fast, the picture resampling is awesome. Pictures are clear. Zoom in and out while viewing is smooth.
The pictures are viewed as thumbnails in folders. Picasa Photo Viewer comes with Picasa 3.6. Try it out even if you've used Picasa before.
The only downside is that it does not animate GIFs, which IrfanView does.