Full Frame simulates full-frame photographic printing, which I've always liked for my printed photos. It even tries to simulate the aberrations on the ersatz enlarger's negative holder. It's not very successful, but I'm working on it. At least it looks slightly more realistic than most of the alternatives.
.scm ﬁle. To install, simply copy to your personal GIMP scripts directory. Under UNIX, this is usually
~/.gimp-2.4/scripts/ (for version 2.4 — every version has its own directory).
Then reload the GIMP or go to Xtns → Script-Fu → Refresh.
The script should appear under Filters → Decor → Full Frame.