Inspired by the Mariners blog Lookout Landing, here's a list of players who might break out (substantially improve on their 2005 rate stats) in 2006. (Note that this isn't the usual definition of "breakout," which is often used in relation to a projection. I haven't done any projections here.) I'll list potential collapse candidates in the next few days. Feel free to add more or argue in the comments.
1. Oliver Perez (5.85 ERA in 2005) and 2. Kip Wells (5.09 ERA). Perez and Wells may have already peaked in 2004 and 2002/2003, respectively, but they're unlikely to be as bad as they were in 2005. They could both break out and still be viewed as disappointments - that is, they could both put up 4.20 ERAs next year and people would still be upset. Perez hasn't done anything too exciting in winter ball, although Dave Littlefield has cautioned writers to not read much into his stats there. The range of possible outcomes for these guys is pretty huge, but I'd bet on Perez falling just about squarely between his 2005 and 2004 stats and Wells being around the midpoint between his 2005 and 2003 stats. For Pirates fans, that won't be good enough.
3. Ian Snell (5.14 ERA in 2005). No, his changeup still isn't good and yes, his attitude is probably terrible, but the extremely low expectations for Snell (from the Pirates, who seem to dislike him, and, anecdotally, from the fans) are hard to fathom. His minor league record is outstanding, and he has produced remarkably consistent stat lines as he has moved up the chain - although he actually markedly improved his control in AAA in 2005. If Snell doesn't improve much, he could still surprise people by putting up a league-average ERA in the rotation; if he defies his reputation for being uncoachable and does improve, especially by learning to set hitters up, watch out.
4. Jack Wilson (.257/.299/.363 in 2005). Wilson is another who could break out simply by not stinking so badly. In 2004, Wilson had 64 extra-base hits. He's not as good as he was in 2004, but those 64 extra-base hits were not an accident - he really was hitting the ball hard in a way he didn't before and hasn't since. Wilson will probably never post a good OBP again, but he could bounce back nicely in 2006 by getting some of that power back.
5. Ryan Doumit (.255/.324/.398 in 2005). He's here mostly because I don't know what to do with him. His minor league record is so thin, and often so good despite his constant injuries, that there's enough reason to hope that Doumit has serious upside. He was also startlingly good at Indianapolis in 2005. In the minors in 2004 and 2005, Doumit had 22 homers and 31 doubles in 386 at bats. It's reasonable to hope that Doumit will start showing some of that power in the majors next year.
Other candidates:
6. Sean Casey. He's also a good collapse candidate because of his lack of athleticism. But if he can overcome his injury problems, he could show more power in 2006. I'm not saying any of this is likely - actually, Casey will probably be a marginal player when he isn't hurt. But keep in mind that Casey hit .381/.534 in 2004. It could happen again.
7. Nate McLouth. Unlikely, but only because McLouth hit pretty well in the majors last year and because he's unlikely to get much playing time unless the Pirates give up on Chris Duffy. Unlike most young Pirates, McLouth has very good plate discipline. He also hit a ton of doubles in 2004, although he only managed 26 between Class AAA and the majors last year. McLouth has the youth and skillset that could make one believe that his power could increase a lot in the next few years. Again, that's not particularly likely, but Jim Edmonds, Brian Giles and Magglio Ordonez all had minor-league records similar to McLouth's.
8. Mike Gonzalez. Not likely, but only because he's already really good. He had trouble with his control last year, though. If he can get his walk rate down to 2004 levels, he can go back to being one of the best relievers in baseball like he was in 2004.
9. Jose Castillo. He made a nice improvement last year and he's not even 25 yet. Plus, he cut his hair, and stuff like that is really important.
From the Minors: Josh Sharpless, Matt Capps and Scott Strickland could all be very pleasant surprises.