I love Google, they get it. Not everyone runs Windows or Macintosh. There are UNIX and Linux users out there too. Besides Google owes a lot to Linux and they know it. So when they make something available to the linux community they don't mess around.
Picasa is a graphics manager. It has some editing abilities but its primarily for indexing and managaging image files. Not only is Picasa an image manager but its free to download and use. Now I am a DigiKam fan and its been my primary image manager for over a year, until now. Digikam, though good, just doesn't have all the abilities and functions Picasa does. Developers, graphic artists and hackers will love Picasa. It doesn't just index graphics files in your home folder, though you can set it to do only that, but it can and will index every image in your directory structure. (If you have a lot of network sharing going on be careful.) If you want to use a specific icon you don't have to hunt your directory structure to locate it just Open Picasa and look for it in there. Want to create your own Icon Theme set using different icons from different sets? Well using Picasa you can do that. Want to locate an elusive image and you have no clue where it is? You can find it with Picasa.
Want to make changes to images in a program? You can because Picasa indexes those images for you so you don't have to hunt them down. AS you can see Picasa is a powerful imaging tool It can't replace The GIMP and as a viewer its a bit overboard but when you need to find a particular image it definitely does the job.
You can move, cut, copy and paste as well. (Though any image or directory protected by root access cannot be deleted, changed, or otherwise altered. You will have to use file and image tools in root mode to do that.) Picasa by default will not show all images but if you go to view on the menu bar you can select Hidden Pictures and Small Pictures. Once those are marked for viewing you see everything, including icons.
Picasa will also handle removable media as well. Since Linux sees digital cameras as removable media devices this is handy.
Now why is Google giving this away? Picasa is a stand alone program but it has built in access to Google's imaging services. They are hoping you will use these services to post, print and order media from them. Services are reasonably priced but Picasa doesn't lock you into them either. You have print and file options as well and of course once you create a photo album with Picasa you can burn your own CD/DVD if you choose.
Picasa itself is not open source though the Linux version does use portions of WINE and Mozilla's Gecko Engine. Open source purists will probably not be interested in Picasa. However, if you, like me, are more interested in functional tools that truly do the job then Picasa is a good choice. Picasa may not be FLOSS but it is free to use.
The Linux version of Picasa can be found here: http://picasa.google.com/linux/