The EZ-Flash V is a Flash Cart for the Nintendo DS handheld gaming console. A flash cart is basically a tool to allow homebrew software, such as MP3 playing software, to be run on your Nintendo DS or DS Lite. Additionally, it can also be used to pirate DS games, but neither the makers of the flash cart or the flash cart community condone this. There are many brands of flash carts available, but I decided to buy this one due to its ease of use and design. Most flash carts run off MicroSD cards; some of the newer models have microSDHC support, and some models use built-in memory. The EZ-Flash V only has support for microSD cards though. This flash cart is extremely easy to use; the most challenging thing to do is upgrading the firmware, and that can be done by downloading a tiny file, uploading it to your MicroSD card, and running it in the EZ-Flash V. There are tons of homebrew software available, such as multimedia players, homebrew games, and even operating systems. Overall, if you are looking to purchase a flash cart, I highly recommend picking up an EZ-Flash V. While it is not advanced as some other flash carts, it is cheap, rich in features, and easy to use.
Update On Jul 31, 2008: Recently, a lot of gaming companies (Nintendo, Capcom, Konami, Square-Enix, just to name a few) have started to oppose these flash carts. For example, when the game "Final Fantasy: Chrystal Chronicles - Rings of Fate" came out, Square-Enix actually had programmed an anti-piracy feature into all copies of the game; it would lock up after half an hour of play time (hackers learned how to disable this feature soon after). So if you're interested in getting one of these things, you'd better do it soon before the makers of these various flash carts cease production due to pressure/lawsuits from the larger gaming companies.
Update On Dec 27, 2008: 2 new models have been released since the last time I updated my review: the EZ-Flash V Plus, and EZ-Flash Vi. The Plus model has support for MicroSDHC cards; the largest I've seen is 16GB. The Vi model works on the new Nintendo DSi (hence Vi), and probably supports MicroSDHC cards as well.