It really started here. But that was a blogpost about this. And it was soon followed by something we perhaps didn't know we needed.
While there are many fine new fielding metrics around using PBP (play-by-play data), to me the really important breakthrough is going to come when we get a reliable metric for fielding in the pre-PBP days. That's going to involve some kind of assumption about a piece of information we'll never have for most of baseball history - how many hits was a fielder responsible for.
I suspect it's quite important to start from the team level when calculating the value of fielding. (See Clay Davenport's article in the 2002 Baseball Prospectus for an example.) You then have a choice - either you can calculate the total of opportunities from the assumption that all balls will fall for hits, and thus the caught balls represent runs saved. Or you can look at fielding in terms of runs allowed, by thinking in terms of the number of mistakes. I suspect for a decent 'all of baseball history' metric, we need to think more along the lines of the latter. We need to establish 'expected runs' baseline against which to measure fielding.
I also came to a key realization after reading all this. Looking at the statistics when I was developing my Defensive Winning Percentage, I was surprised at how narrow the band is of fielding effectiveness. This is, I think, a reflection that there's both a floor and a ceiling to the effectiveness of fielders. Some of the credit is all down to the pitcher. All home runs except those inside-the-park, for example, are the pitcher's fault. Only some of the runs scored by those who walk, however, are the pitcher's fault. So there's some fielding input on that. But a walk and home run equals all the pitcher's fault.
Probably, the Fielding Experts will say I'm not being very original here. But I hadn't thought of it that way before.