Bucket Brigade Devices (BBDs) are integrated circuits which sample and delay an input signal. These interesting devices were invented in the late 1960s and are a rare example of a discrete-time, but non-digital (ie, continuous-valued), electronic system. In order to reduce aliasing, distortion, and noise, BBDs are typically accompanied by low-pass flters and compander circuitry. Through circuit analysis and measurements, each component of the typical BBD system can be accurately modeled. I wrote a paper on the subject of creating accurate software simulations of BBD systems for the 2010 Digital Audio Effects Conference in Austria, which you can download here.
An example of a modeled bucket-brigade echo system using the Synthesis ToolKit can be found here. A Matlab script for generating filter coefficients based on component values used in a BBD-based circuit's filters can be found here.