Atmel recently came out with app-note AVR2070, which details using Atmels 802.15.4 radios (RF231/RF212) with a free software stack. The stack provides a self-healing and self-forming network, called Route Under MAC (RUM). See www.atmel.com/6lowpan for the PDF and to download the software.
The stack can take as little as 6 KB of ROM, so takes very little resources. The SAM7X256 board can be used as a coordinator - with this option 6LoWPAN can be enabled, which means all nodes can be accessed using IPv6. The SAM7X board runs a webserver which allows you to view and configure the network, and addition options can be configured through telnet. Of course just an AVR node could be used as a coordinator "“ no need for a SAM7X board if you don't have one!
You do need a SAM7X256 for 6LoWPAN "“ but hopefully soon some new hardware will be coming out which is a bit cheaper.
If you have more questions about the stack post them here!