1:26pm: According to SI.com's Jon Heyman, the Yankees and Giants have also begun showing serious interest in Penny. Heyman opines, and many others agree, that an "NL team might be better for him."
9:19am: Jon Paul Morosi at FOX Sports says that the Marlins are "among the most serious suitors" for Brad Penny's services in the wake of Penny's release from the Red Sox this week. Penny is apparently set to clear release waivers sometime soon.
The Marlins' interest has been noted here, but it looks like a decision will come soon. Morosi believes that the finalization of the move will probably be predicated on how well the Fish play against the Padres this weekend, and says that winning this series could justify a Penny addition. Morosi also says that the Rockies, while still looking for starting pitching, aren't in the market for Penny.
When Penny departed the Marlins for the Dodgers in 2004 in a trade he was in top form, having put up a 3.15 ERA and 105/39 K/BB ratio in 131 innings that season. He's only matched that brilliance in one season since, but a move to the NL certainly can't hurt.