Take one Nintendo DS, a handful of solar panels, some spare time and this Instructable, and you could end up with this: a solar-powered DS that you never have to plug into the mains again. By strapping four 60 x 60mm panels - each capable of a maximum 3 volts at 40 ma power output - to the top and bottom outside casings of the DS, you can actually create a trickle-charger potent enough to avoid ever needing mains power.
The original maker, dark sponge, claims to have never needed the PSU since fitting the panels. Obviously your mileage will depend on how much you use your DS versus how long you leave it soaking up the sun, but it's such a cheap hack that even the heaviest of gamers would save at least a few dollars over the year.
The solar-panels themselves cost around $3.75 each, making this one of the most impressive, eco- and wallet-friendly DS mods we've see to date. If I had a DS, and didn't live in London where the sun seems to have been permanently banned, this is one hack I'd actually attempt myself