I built a simple system using zigbee radio modules to monitor the output from a solar panel;
it's described here: http://chezphil.org/slugbee/ - that page also has links to all the
zigbee modules that I considered at that time.
I used modules from DLP; the "base station" is connected by USB to an NSLU2 running Linux, and
at the remote end the module has a handful of analogue and digital IOs. It's good technology.