The $US50, 4GB MAX Memory card from Datel for the Xbox 360 is special not because there aren't many third-party Xbox 360 memory cards around, but because it supports microSD cards.
With the microSD cards (up to 16GB), you'll be able to supplement your own Xbox 360 expandable storage without having to pay the exorbitant rates Microsoft is charging for its own memory cards. $US42 for 512MB, Microsoft? Really?
Plus with the Datel, you can dump the contents of your card directly onto your PC. Supremely useful if you have a load of games that you may or may not get back into at a later date, but want to keep the saves around just in case. [Code Junkies]