SparkFun has mounted a RFM22 433MHz ISM FSK transceiver module to a shield. What's it do? Let you do a cheap point-to-point (and basic mesh networks) using 433MHz!
RFM22 433MHz ISM FSK transceiver module. What does that mean for you? It means your Arduino now has an inexpensive way to talk to other Arduinos so building point-to-point and even basic mesh networks just got a lot more attractive.
Use the RF22 Arduino Library, you can send and receive messages via standard 4-wire SPI. Wirelessly. From/to the other module! The library includes command structures for setting up anything from simple non-addressed point-to-point communication to fully addressed networks of clients and routers. The shield also provides a through-hole antenna connection for hooking up a basic wire antenna. In our testing, a 17cm wire attached to the 'ANT' pin worked just fine. A hole is drilled next to the 'ANT' pin for strain relief.
The shield has on-board power regulation and level shifting. There are one pin headers and jumpers on the interrupt (nIRQ), interface select (nSEL), shutdown input (SDN) and GPIO2 lines so that you can cut the trace and reroute those lines if you need to for your project. The board also provides a little bit of prototyping space.