Microsoft's storage solution - $20 for a 20GB HD or a free 512 MB card

[matt247]

http://www.xbox.com/en-US/support/hardware/memoryupdate/default.htm
http://www.xboxstorageupgrade.com/hh/web/home/hhauthentication.jsp?ssLocale=en_US

Edit: The HD is refurbished, but it's still a great deal and only core/arcade owners are eligible