If you're a Flickr user, then you'll appreciate the immense resource that the service is for finding images. Viewfinder aims to make the process of finding, downloading and using images from Flickr as simple as possible.
There are an estimate 4 billion photos on Flickr and Viewfinder provides a one-stop interface for searching tags, titles and descriptions. You can sort them by most interesting, most relevant or most recent. Viewfinder is also handy for filtering by license - you can select images with Creative Commons licenses and if you want to use a photo for something, Viewfinder will generate the appropriate copyright attribution for you.
Searching is very fast and if you're not satisfied with the number of results returned, a 'Load More' button retrieves more images for you. The preferences are very limited - you can only specify the download location and auto-update feature. There's also a handy Keynote button which sends pictures to the Keynote '08 presentation app.
You can also select to open photos directly in Flickr or set them as your Desktop Background. Some photos however are protected by the owners and Viewfinder won't let you download them at all.
Viewfinder is a handy app for any Flickr fan although any future changes to Flickr's API will no doubt affect it's functionality.