Crossbow – we are using this product at Arup – the Mica2, Mica2 dot and more recently the Micaz. Gateways include the MIB510 (with USB adapter) and the MIB600 ethernet gateway. Sensor boards include the MTS300 and 310’s, the MTS420 and the MDA500. We also have the Moteworks software.
Whilst not strictly WSN technology (but you can get zigbee and bluetooth add ons) we are also using Arduino boards to prototype sensor and actuator interfaces.
Also of interest are SunSpots when they become available.
Others such as Urban Tapestries project are also using MoteIV as an alternative platform.
The DTI Global Watch Mission to US in late 2005 – pdf report