The Picasa is with a free full version of innovations that make it a complete application for viewing photos and images.
Packed in a compact size, this software provides a lot of features while being fast and responsive. I have been using the Irfan view since my college days. IrfanView is one of the most popular free-to-use image viewer and editor.
XnView MP is commercial software by XnSoft but you can use it completely for free if you want it for personal use or for a non-profit organization.