I was playing with GIMP the other day trying to find some filters or scripts that created nice borders around images. I use Digikam to organise my collection of photos and it has some built-in tools but the after effects are rather simple at the moment. GIMP has a more mature collection of effects to apply.
Taking a sample image I applied a selection of the decor filters to the images. You can find these under Filters > Decor. I am using the latest version of GIMP on Linux Mint 10 KDE and Windows 7 Pro.