Nintendo's 3DS is an impressive handheld gaming device, but maintains a less welcome gaming tradition: it's region-locked, so you can't buy games from overseas. If that's an issue for you, it's possible to hack your 3DS to remove the region lock.
The hack, detailed at the GBAtemp forum, doesn't require any particularly complex techniques: all you need is a spare 3DS flashcard. One restriction: it only works on version 4.1 to 4.5 of the 3DS firmware. Version 5 was released in April 2013 and the software is now up to version 7.1, so if you've upgraded in the last year you're out of lock.
As with any firmware hack, this is at your own risk: if something goes wrong, Nintendo isn't going to help you get it fixed. As Mark at Kotaku points out, it also won't enable you to play illegally-downloaded games: it's purely designed to ensure that you can use games purchased in any country.
Region Free Cart Loading with EmuNAND No flashcards [GBAtemp via Kotaku]