This Blaze Game Selector for the Nintendo DS Lite attaches to the DS's back and allows you to toggle between 3 games with the flip of a switch. We're not sure the marginal effort saved is worth the aesthetic sacrifice.
The Selector features three slots for games and an indicator light to show which game is in play. A cable runs between the attachment and the DS's card slot, and it looks like the Selector is easily removable, as it just clips on. It's mildly useful, especially if you own exactly three games, but it really ruins the sleek lines of the DS Lite, and, of course, it's vastly inferior in utility (though not in legality, heh) to one card with boatloads of ROMs. The reviews are glowing but we're not sure they're trustworthy, especially the one saying, "I really enjoy it and look forward to doing it over and over. Well worth it."