Arduino

Arduino is an inexpensive and open source hardware prototyping environment. It can take input from any number of sensors, and output to devices, actuators, computers, etc.

A new Bluetooth module is just out - this is Class 1 Bluetooth, with range of about 100 meters (versus 10m for normal Bluetooth). Up to 7 devices can connect via Bluetooth; this can be extended with one of several hacks such as a 'scatternet' in which one device bridges two networks; or by automatically connecting and disconnecting.

Telit makes a GSM modem module which provides onboard mobile telephony. With an audio chip it could make phone calls; it has a microphone input. So effectively you could build your own mobile phone if you want to. It can also do SMS, email and FTP, has an embedded TCP/IP stack, and Python interpreter.

Arduino is also working on an embedded web server on the board, so you can check and set inputs and outputs to a remote module from a web page.

A ZigBee module is also under development.

Pure Data is a free and open source visual programming environment (like Max/MSP) for which a graphical Arduino programmer has been written.