If you have an early car, you might have heard the "Auxiliary Fuse Box" mentioned around the forum. The box slips onto the side of the main fuse box and has a red and green wire. The red connects inside the box at C100 and the green at C200. The red provides constant-on and the green is switched through the ignition switch. Included are two 25a fuses and the cover, which compliments the main box swimmingly.
These are very useful for aftermarket stereo installations, where you'd prefer to have a proper, OE look.
I have another. This one is from a Super Eta and doesn't have the bracket to slide onto the side of the fuse box. Works the same way, though. Same price, too...