Pinball.
Thanks to R. Belmont, you can now play music directly from arcade ROMS with a little program called M1. It's based on MAME, and is extremely simple to use, especially if you also get the separate frontend BridgeM1.

You can use it just as a player, or you can record the output to WAV files so that you can make your own music CDs.

While you're getting BridgeM1, make sure to also get the LIST files. These are song lists made by a very dedicated group of people at e2j.net, and make the games much easier to navigate through. An example of the frontend in use with lists is here to the right. If a list doesn't yet exist for your favorite game, you can also make your own; just refer to the documentation and the other lists as a guide.

If you already use MAME, just point M1 to your MAME ROM directory, as over 90% of the currently supported games are the same sets that are supported in MAME. M1 also supports some Pinball ROMS, and there are also few games in M1 that are not currently supported in MAME, which you will have to get separately.

To call it a fun toy would be a misrepresentation. It's a work of art, and I, for one, am glad that RB was inspired to create such a project.