Picasa was originally available as Windows PC software but Google decided to release a Mac version.  A Linux version has also been available for some time.

Google makes the following statements in the Google Photos Blog:
'Picasa for Mac is a standalone program that helps you organize photos anywhere on your hard disk, edit your photos to perfection, and then easily share them online.'

'Picasa for Mac looks and works almost exactly like Picasa on other platforms. It can keep track of photo files scattered across your hard drives, and will automatically account for new photos as you add them to your system. Picasa for Mac also features non-destructive editing, so you can explore different photo adjustments and effects without worry.'

'Picasa for Mac is initially being released as a Google Labs product -- it's very much a beta. Some smaller features like geotagging and 3rd-party printing aren't functional, yet, and you may well stumble across other rough edges as you use this beta.'

Picasa for Mac includes

• A drag-and-drop photo collage tool that gives users control over layout and content
• A retouching brush to wipe out scratches and blemishes - and repair old photos
• A slideshow movie maker that uploads users photo montage videos to YouTube with a click
• Smart auto-cropping that guides users on how to zoom in on their subject
• And, auto red-eye removal.

System Requirements

• Mac OS X 10.4+
• Intel CPU
• 256MB RAM
• 100MB available hard disk space

You can get it at the following download page.  Find out more at the Google Photos Blog post and Press Center article.