This one from an interesting 82games.com column.
They've compiled the +/- for everyone in the league and ranked them by on/off court total - which I'll let them explain.
There are three Mavs in the top 25 and Devin Harris leads them all at 7th in the league. Over 48 minutes, the Mavs are +12 when he's on the court and -8.3 when he's off. That gives him a +20.3 overall rating. Dirk is not far behind at +18.4, and Jason Terry is slightly behind him at +15.9.
At the other end of the spectrum is Anthony Johnson with the 17th worst on/off rating in the league. Dallas is -5.9 with him on the court and +8.5 with him off for an overall of -15.1. Of course, Anthony Johnson is far from the 17th worst player in the league and this where some of the flaws of this stat begin to be seen.
But that just brings me back to Devin Harris. Unlike Anthony Johnson, Harris spends most of his time on the court with Terry and Dirk and very little with players from the end of the bench -- his +/- certainly benefits from that. But the same goes for Terry, Dirk, and Dampier and he leads them all.
Hardly the ultimate statistic, but something to think about.