Nick Swisher and Austin Kearns would be on my list. I could see Arroyo having a year similar to his 2006 numbers. Whether he ends up as a pitcher or outfielder, I think Micah Owings could be a huge sleeper. John Maine might blossom into an ace, but I guess he's not really a sleeper. Nick Markakis isn't exactly a sleeper either, but I think he could go from 'really good' to 'superstar'. J.R. Towles was a big flop for the Astros, but he could come back in a major way. A lot of people are already writing off Randy Johnson because of his age, but going to San Francisco could help his numbers quite a bit. Max Scherzer could be another ace in the making.
Anyway, I guess my #1 sleeper would be J.R. Towles. He might enter himself into the Russell Martin/Brian McCann group. #2 would be Scherzer just because he's so freaking good already.