Design News posted an introduction to the Bus Pirate with a short review:

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I think Bus Pirate is an interesting project and a very useful tool to have available at the price.  I think a great addition to this project would be a GUI front end that would allow you to point/click to configure it and create macros, logging all of the commands used so that once completed the log can be reused as a script in the future.

A commenter picked out the primary design goal for the Bus Pirate concept:

There are other FTDI-based USB adapters/custom boards that provide direct drivers but additional programming & design is required (on both the PC side and the PIC/USB side).

There’s lots of USB->serial protocol devices out there. Most of them offload programming from a microcontroller to a custom Windows DLL. You’re still stuck writing code for an unknown device, but with an additional layer of obfuscation and unknown errors. The Bus Pirate concept is the opposite of high-level abstraction. It’s a tool to learn about a chip interface at a very basic level.

When we sit down with a new chip, we open the datasheet on one half of the screen and a serial terminal on the other. We use the Bus Pirate to play with the chip functions while reading through the datasheet, without writing code or burning firmware. Most of the Bus Pirate source is public domain, so you can use the proven code in your final design without any restrictions.

You can buy Bus Pirate v3 for $30, including worldwide shipping. The most recent preorder (preorder 3) should start shipping this week.