• Cole Hamels, Phillies – Hamels is one of the best pitchers in the game. Though he's just 28, he has a seven-year history of success in a major market, including a World Series MVP award. Hamels has the potential to improve some team’s rotation dramatically and demand will reflect that upside if the Phillies make him available.
• Zack Greinke, Brewers – If you go by the numbers, Greinke's just as desirable as Greinke. However, some teams question Greinke's ability to thrive in the spotlight. Still, the right-hander has the potential to make a team’s front office look really smart.
• Matt Garza, Cubs – Garza's 4.32 ERA is pretty ordinary, but he’s still a tremendously valuable trade chip for the Cubs. Unlike Greinke and Hamels, Garza's controllable through 2013. He has succeeded in the AL East and his peripheral stats remain strong.
• Wandy Rodriguez, Astros – Rodriguez has a 3.38 ERA through 18 starts, but his salary could be an obstacle for some clubs. The left-hander will earn $10MM in 2012, $13MM in 2013 and, potentially, $13MM in 2014 (the team’s ’14 option becomes a player option following a trade). However, the Astros showed that they’re willing to absorb salary if it means a chance at obtaining better prospects when they dealt Carlos Lee to the Marlins.
• Ryan Dempster, Cubs – Dempster, who has an NL-best 1.99 ERA, should draw lots of interest as a mid-season trade candidate. The Cubs seem willing to absorb some of his $14MM salary to facilitate a deal.
• Shaun Marcum, Brewers – Marcum, now on the disabled list, is on track to hit free agency this offseason. He has a 3.39 ERA with 8.4 K/9 and 2.8 BB/9 through 82 1/3 innings in Milwaukee.
• Paul Maholm, Cubs – Maholm could appeal to teams looking to add back-of-the-rotation depth. His contract includes a $6.5MM option for 2013.
• Randy Wolf, Brewers – Wolf's trade value would be limited by his 5.80 ERA and $9.5MM salary.
• Jeremy Guthrie, Rockies – Guthrie’s trade value may be at an all-time low following three disappointing months in Denver.
• Edinson Volquez and Clayton Richard could be available, but there’s no reason for the Padres to trade such controllable, young arms without obtaining a remarkable package in return.
• Jason Vargas, Mariners – Vargas is similar pitcher to Wandy Rodriguez, but he's younger and less expensive, as I explained last month.
• Kevin Millwood, Mariners – Millwood, 37, has a 3.69 ERA with 6.9 K/9 and 3.5 BB/9 in 90 1/3 innings with Seattle. He earns $1MM and will hit free agency after the season.
• Despite the annual speculation surrounding Felix Hernandez, there’s no indication he’s truly available.
• Bartolo Colon, Athletics – Colon, 39, is the player the Athletics are most likely to trade, according to Ken Rosenthal. The righty has a 3.80 ERA with 5.4 K/9 and 1.5 BB/9 through 104 1/3 innings.
• Brandon McCarthy, Athletics – The A's would reportedly consider offers for McCarthy, a prospective free agent who has a 2.54 ERA with 6.0 K/9 and 2.2 BB/9. He's now on the 15-day disabled list.
• Brian Matusz, Orioles – Matusz appears to be a trade chip for the Orioles. He's now in the minor leagues after posting a 5.42 ERA through 16 starts in Baltimore.
• Francisco Liriano, Twins – Liriano, a free agent after the season, has pitched well since returning to the rotation. Still, clubs are wary of trading for the left-hander, whose walk rate remains high.

MLBTR has previewed the trade market by looking at potentially available catchers, first basemen, second basemen, third basemen, shortstops and outfielders. Left-handed relievers are up next.