http://perso.wanadoo.fr/pierre.g/xnview/enhome.html
Very nice alternative to Irfanview, with built in image browser (like ACDSEE).
It also has versions for PocketPC, MAC, Linux.. in fact pretty much any operating system you can think of.
It supports skins too..

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After using it for a few days.. it's rapidly becoming my preference over Irfanview.