Most Underrated: Tom Gorzelanny
One of the most underrated pitchers in the hobby is the Pirates' Tom Gorzelanny. Overall this season, he's 14-7 with a 3.48 ERA (for a team that's 62-79; 2nd worst in the NL). His ERA is better than some of the hotter pitchers out there, like Chien-Ming Wang (3.68), Roy Halladay (3.94), Justin Verlander (3.56) and Carlos Zambrano (4.35), but his rookie cards don't even come close to these players' card values. This year, he's allowed 3 or fewer runs in 21 of his 28 starts (2 or fewer runs in 16 starts).
His best rookie (in my opinion) is his 03 UD Prospect Premiers Auto, which was selling in the $14-15 TAC range through June & July, but fell to $10-11 in August, and now can be found for less than $10.
The other rookie-year Auto that I've seen is his 03 Bowman Heritage Auto, which can be found in the $7-10 TAC range.
Gorzelanny is having a break out season, and hopefully it's just the start of a solid career.