Probably alot of you know what the Raspberry Pi is - a US$35 micro computer the size of a credit card that can be used for alot of simple but cool projects. Well, some guys at the Imperial College Robotics Society in UK found out that you can actually output a modulated clock signal on one of the GPIO pins to act as a small RF transmitter, then Dan MD1CLV and Guido PE1NNZ took the idea further and implemented the necessary coding in order to make it a working WSPR beacon - project's name is WsprryPi .
Considering the limits for the GPIO output (TTL 3.3V @ max 50mA) we can't expect too much output power, 10mW (+10dBm) into 50ohm is a reasonable figure while about 20mW is the technical limit. One issue would be the fact that the output signal is a square waveform therefore rich in harmonics and a low-pass filter for each transmit frequency is mandatory (especially if you want to boost the power a bit); considering it covers from 0 to about 250Mhz, if you plan to make a multiband beacon out of it you will need alot of filters. At the moment it needs a permanent internet connection for time synchronisation, but i'm sure some handy fellow will implement a GPS time reference to make it completely standalone.