Picasa 3 is the rich photo management software from Google.Picasa photo viewer is a fast, lightweight app for viewing images on your desktop or through Windows Explorer.A quick double-click on any photo will give you a full-screen view and few options at your fingertips: you can add stars, zoom in and out with your mouse scroll wheel, rotate, or open photo in Picasa 3 for further edits. By any means this feature works much better than standard Windows Picture and Fax viewer.

Features :

Organize

Manage your photos in one place, and find photos you forgot you had Edit

Eliminate scratches & blemishes, fix red-eye, crop and more Create

Turn photos into movies, collages, slideshows and more Share

Upload seamlessly to Picasa Web Albums to share with friends, family & the world

You can see the full list of Picasa 3 futures here.

Screenshot of Picasa Photo collage :

Supported File Formats :

It supports image file formats like .JPEG, .TIF, .TIFF, .BMP, .GIF, .PSD (Photoshop), .PNG, .TGA, and some RAW formats) and video (AVI, .MPG, .ASF, .WMV, .MOV, and 3gp.

My personal opinion is Picasa 3 is really better than ACDSee and other photo editing softwares.

Links : Download Picasa 3 | Full List of Features | Picasa 3 Homepage