Although mentioned in another thread, I thought it might save a few people some money if, when they're looking for a simple RF data link, they consider the Telegesis ETRX2 modules.

These use a self healing mesh networking topology so you can introduce as many nodes as you wish to your network and once created by you the network manages itself.

These particular modules have inbuilt digital I/O and A/D, so,
using a simple remote temperature logger as an example, we can do away with the remote host processor completely and replace the RF Module+PIC combination with a single ETRX2.

Sending the following command to the distant ETRX2 from the main coordinator (and data sink) makes the remote send back data relevant to the I/O pins and the A/D

ATREMS:<address>,XX?

Where <address> can be the remote node’s EUI64, NodeID or address table index and <XX> is the register number relating to the data.

All commands are in the AT format so communicating via PICBasic and a PIC is made easy.

My units cost £11.90 each and I suppose if you were making a simple 2-node network then you might be able to beat that price (especially if you are operating in simplex mode), but I feel the ETRX2 are more reliable and less noise prone than other RF modules I have used before. The units have a feature where they can actively search for the quietest channel, and data transmission can be monitored and the creation of ACK and NACK messages is automatic, which means that we can have some reassurance that a message was delivered.

I don't work for Telegesis, but recently have tried several Zigbee modules and development kits and this was by far the easiest way to go
http://www.telegesis.com/products/test_page_1.htm

The full list of commands is on the document download page in a document called "R3xx command list"