are you printing them? yourself? or just for computer use? or what?
ps elements works... not sure of a way to do it in iphoto or any other such app. i bet gimp or other various freeware can do it though.
if you're talking of printing it, you can sometimes get online printing services to add borders for you, which may be a better option. or, could get a matte if you plan on framing... may look better than a bunch of black (or whatever color) ink.