I have created a digital loop pedal. The pedal itself is based upon the PIC16F877 microcontroller, and is complete with 256kb of RAM for storing sound. The unit achieves true bypass with the help of a relay, and runs on a standard 9V power supply.
If you want to build a pedal like this, be warned- it is a daunting task. The complexity of this project reached the upper limit of what my brain can autoroute on a prototyping board, and was very frustrating many times when it didn't work as planned.
In the end however, I learned a lot. I think this is a great project to try, but only try if you are really dedicated. For those interested, I would suggest you design your own, using mine as a template to know what works and what doesn't, rather than copying directly- though if you wish to copy, you are more than welcome.