pocket NES
Flash memory cards are not uncommon in the console world; I remember my friend using them to backup savegames on his Playstation back in the day. So it should come as no surprise that there are flash memory cartridges for the GBA. I found out about them through the Pocket NES project. It's a great idea, even though it involves using the ROMs of NES titles, which is not exactly legal in the fullest sense of the word. Well, unless you actually own the original cartridges, in which case ROMs are actually backups. Most of the kits that you can buy come with software, USB and serial attachments that you can use to upload to your GBA from your PC. The only downside of these memory cartridges is that they're pretty expensive. You're looking at about $200 CDN for a 256 MB cartridge alone.
I'd love to be able to play some of my favorite NES side scrollers like Castlevania III, Batman and Contra on my GBA. In fact, what would be even cooler is if someone were to write a Genesis emulator for the GBA, so I could enjoy my favorite 16-bit game of all time: Shinobi III: Return of the Ninja Master. You laugh maybe, but I'm telling you it was the greatest combintation of action, adventure and music. The soundtrack was incredible. The exotic instruments of the mysterious Orient meets western guitar. The Neo Zeed didn't have a fucking chance.
On that note, I should say that I'm a big supporter of abandonware. I use emulators for the old systems and SCUMMVM to play the classic LucasArts games. I mean if you can't find an old game in any store and don't have the original game system, why shouldn't you be able to play them? It's not like anyone would pay money for them; they're old. Sites like The Underdogs are doing this for PC games. Stuff that you couldn't possibly hope to find except maybe on ebay is available for you to check out. It's a great way of preserving the history of videogames.
During my search for third-party GBA accessories I also came across a retail TV tuner available for the GBA. It doesn't look too bad, although it seems to be designed for the original GBA. I'm not sure if the tuner is official, though, since it requires a game to be in the GBA for it to work. Apparently it hacks into the game's boot sequence so that the tuner can take control of the control pad and screen. You could also plug your DVD player into the tuner, if you were so inclined. This to me is kind of pointless though, when compared with the movie players that will soon be available.
