Picasa is an excellent and functional image editing application that is available free from Google.
The photo editor allows you to make basic lighting fixes, crop and straighten photos, adjust colour and add a selection of effects such as soft focus, glow, black & white conversions and graduated tints.
I often find myself firing up Picasa rather than Photoshop to make quick, on-the-fly adjustments.
The program can also catalog all your photos and is very easy to use.
Picasa is a small 14MB download, available from picasa.google.com.