I purchase a Bus Pirate from Sparkfun last week and have to say that it's going to be an excellent tool for prototyping and testing. From Sparkfun: the Bus Pirate "communicates between a PC and any embedded device over most standard serial protocols, which include I2C, SPI, and asynchronous serial - all at voltages from 0-5.5VDC". In short, I can connect my bus pirate to almost any I/O interface and read/write from/to it.

Here's the Bus Pirate connected to an RFID tag reader, and connected via USB to my PC on the other end. I telnet into the Bus Pirate, choose the protocol and settings that I wish to use (UART, in this case), and can then manually read and write against the connected device. In this instance, I receive the string value of the RFID tag every time is is swiped.

In the terminal, you can see I've telnet-ed into the Bus Pirate's interface, selected interface settings, then passed some commands to the RFID tag reader. "W" turns the 5V power out to ON. I then set the AUX pin to LOW, which enables the reader. At the bottom of the terminal, you can see I swiped two different tags, and receive the output from the tag reader back in real-time.