While Alex Mack is regarded as the best center on the market, He's not likely leaving Cleveland after the Browns slapped the transition tag on him.
That left Brian de la Puente as the best free agent center left on the market, which he's on no longer after inking a deal with the Chicago Bears:
He's started the past 32 games for the New Orleans Saints and was though to be close to re-signing with them, but the Bears were able to swoop in and sign him away.
Jonathan Goodwin is now the best center on the market, and the Saints have been rumored to be a team interested in him. He's played in started in every game for the San Francisco 49ers over the past three seasons.
Prior to that, Goodwin played and started all but three games for the Saints form 2008-10.