My apologies if this has been covered before, but I thought this was a great deal and wanted to contribute a little bit.
What is an R4DS?
It is a way to play backup games on your Nintendo DS. It looks just like a standard DS cartridge, and its really easy to use!
Where can you get one?
The best deal I've found is at dealextreme. Only $30.68 with free shipping.
Note: The shipping comes from China so it may take a while, but its the
Do you need anything else to run backup games?
Yes, you need a micro SD card to put into the cartridge for memory storage. You can get a 1GB one from meritline.com for only $12.95 Shipped
Frys has a 1GB microSD card for $9.99 Shipped!
Here is where you can get the latest firmware: