Picasa (web-Latin for 'Picture House'??) is a great picture organising tool by Google, with some very simple but useful picture editing tools (straighten, crop, lighten, darken, saturate, black-and-white, etc).
It preserves all your pictures on your hard drive, making back-up copies of originals whenever you make editorial changes, so all your pictures are safe. It makes browsing and organising your images a breeze.
You can produce some great visual effects, as well as 'rescuing' flawed photos. And it's heaps easier to use than Photoshop, though I do often work between Picasa and Photoshop - while Photoshop wins hands down for tasks such as layering, masking and spot-healing, some of the Picassa tools (eg, straightening/croping, lightening/darkening) are so much quicker and easier in Picasa and can create a better effect.
And it's FREE - did I mention that?!