Two of the most undervalued players I've seen are Nick Swisher and Kelly Johnson.
Swisher hit 35 HRs in Oakland a couple years ago and had a down year last year. But now he's in one of the best HR parks in baseball and he'll be protected by Thome, Konerko, and Dye in a much, MUCH better lineup. Better pitches to hit, more runs being scored, and lots of potential to be a huge steal for where he's being drafted. I won't be suprised when he ends up with a batting average around .275 with an OBP over .400, and I'll be shocked if his power doesn't at least return to where it was in 2006.
And I love Kelly Johnson too. With full-time duty, he's got a shot to be a top 5 2B hitting leadoff in that lineup. 20+ HRs, double digit steals, a batting average near .280 and an OBP 100 points higher. And he's only going to get better now that he doesn't have to learn how to play 2B anymore.