These work okay-ish across some number of feet. You need one per Arduino, at $5.50 each:
http://iteadstudio.com/store/index.php?main_page=product_info&cPath=7&products_id=53
These are the same thing, but with an actually implemented RF end, so they go several hundred feet, even through walls:
http://iteadstudio.com/store/index.php?main_page=product_info&cPath=7&products_id=419
I have used them and can verify that they work more or less as advertised, although the available sample code is somewhat bad. I ended up writing my own driver by reading the data sheet, but the included samples can work, too.
Note that the advertisement talks about a 250 kbps transmission mode to get to 800m range -- that's a copy-and-paste error. The chip can only do 1 Mbit or 2 Mbit, and the 1 Mbit mode goes a few hundred feet (call it a hundred meters.)