The NYTimes also has a fascinating article on a sabermetric publication known as the PECOTA. It basically projects the possible trajectories of each player for the upcoming season and assigns probabilities to each.
...Take Johnny Damon, the Red Sox center fielder considered one of this off-season's most attractive free agents. Pecota examines many factors beyond his raw statistical record - the effect of playing in hitter-friendly Fenway Park, Damon's age (32), power and speed, even his height and weight - and compares him with every major leaguer since 1949, identifying the trajectories others have taken and assessing the probability Damon will follow them.
The system forecasts Damon to have about a 25 percent chance of posting an on-base plus slugging percentage (OPS) of .829 next season, but also a 40 percent chance of declining to .761. This is because players like Damon have maintained their performance only for about one year before beginning a consistent, decided decline. In other words, buyer beware.
Pecota is less pessimistic toward shortstop Rafael Furcal, who at 28 has a better chance of maintaining his performance level. (Though the system suggests that middle infielders age more quickly than classic sluggers.) A relative sleeper could be found in outfielder Brian Giles, who despite being 35 has the kind of skills - excellent power, speed and a fine batting eye - that tend to age relatively slowly.