Marcel is a system of player projections developed by statistician Tom Tango. It is one of the most basic systems of player projection available. In fact, Tango himself describes Marcel as "the most basic forecasting system you can have, that uses as little intelligence as possible."
The name Marcel is actually short for "The Marcel the Monkey Forecasting System," with the monkey being a metaphor for the basest system of statistical projections available. According to Tango, the system simply, "Uses three years of MLB data, with the most recent data weighted heavier. It regresses toward the mean. And it has an age factor."
Marcel also incorporates a "reliability" factor for every player. For players who have more data available, that reliability factor is higher. Meanwhile, younger, untested players typically have low reliability factors with projections very close to the league average.
Marcel has frustrated many a predictor because - despite the simplicity of its methods -- it typically performs on par with other more complex systems.