"Blorb" is a nonsense word from a popular early 1980s work of IF called Enchanter where it was the name of a magic spell whose purpose was to "safely protect a small object as though in a strong box." In the late 1990s, the name was borrowed for a standard format for what might be called the wrapping and packaging of IF. A typical Blorb archive produced by Inform contains the "story file" -- the actual program for the game -- together with its library card and cover art.
-- Graham Nelson in Writing with Inform
This program uses that cover art to generate thumbnail previews for GNOME's file manager. It works with Nautilus (in GNOME 2 and GNOME 3) and Unity (in Ubuntu 10.10+).
The heart of the program is a command-line utility (blorb-thumbnailer) that relies on the GDK-PixBuf library to load, scale, and save the cover in question. It's accompanied by helper scripts and format specifications that tell Nautilus (the file manager) how and when to invoke it.
Like GNOME, it's free software released under the GNU GPL.
To compile, you'll need the GDK-PixBuf development headers. Lately they've been given a package of their own -- something like libgdk-pixbuf2.0-dev or gdk-pixbuf2-devel -- but in the past they've been bundled with the rest of the GTK+ headers (e.g., in libgtk2.0-dev).
In version 1.3 (March 2012):
See also the repository for any further developments.
For blorbs that don't have covers, and for games in other formats, try IF Icons.
For more fun with blorbs, see Babel, cBlorb, and blorbtool.py.
For putting together an iTunes-style collection, check out Grotesque.
And for a similar thumbnailer for Mac OS X, look into Bruegel.