There's the old cliche that production is just a function of talent and opportunity. While there's usually a bit more than that, I like to base my sleepers on those two points. Also, the majority of these players are going to be veterans who have been given short shrift recently. The reason I gravitate towards vets is that they are PROVEN, and their upside is almost always underrated. People go after the flashier picks, the sexier picks. Drafters like guys with good measureables, or who were big news recently. As a result, a lot of veterans who are very clearly poised for fantastic seasons fall through the cracks.
QB-
Jake Plummer- Usually, when it comes to QBs, you have three main tiers. There's the "elite" tier, which is one or two (or occassional 3) QBs who are the unquestioned, indisputable studs. Peyton Manning is always in this tier. Culpepper and McNabb used to be in this tier before they got injured, and Palmer would be there if not for the injury, too. The next tier is the very solid QBs without a lot of questions surrounding them. This year, that would be the "Brady/Hasselbeck" tier (some people, including me, put McNabb here, too). After that, you have a massive third tier (usually around 10 QBs) where the difference between the top guy and the bottom guy is really inconsequential. As a result, the best value usually lies right at the end of that third tier. This year, Plummer seems to be one of two or three QBs who lives at the end of the third tier (along with Brooks and Trent Green). In my opinion, that's absolute lunacy. Plummer was the #5 fantasy QB two years ago, and was the #11 fantasy QB last year... and between the loss of Anderson and the addition of Walker (and possibly Scheffler and Marshall), Plummer's situation has SIGNIFICANTLY and DEMONSTRABLY improved. In my opinion, there is absolutely no reason he should be going as low as he's going. I have him ranked in my top 6 QBs this season, and he can usually be had as the 12th QB off the board.
RB-
Thomas Jones- Thomas Jones is every bit as talented as he was last season. Chicago's offense will afford Thomas Jones every bit as much opportunity this year as it did last year. Cedric Benson pretty much hasn't practiced all preseason, and hasn't gotten a single preseason carry. Thomas Jones was the #9 RB in the NFL last year. Thomas Jones is being drafted as the 27th RB off the board, a mere THREE SLOTS ahead of Benson. If you're really concerned about the situation, burn a pair of picks to grab both backs on the turn. Otherwise, grab Jones and laugh your way to the bank.
WR-
Lee Evans- He has already proven, in my mind, that he's a great talent. He had the best rookie season of anyone not named Moss, Boldin, or Clayton, and he followed that up with a very solid second season, all things considered. He's now the unquestioned #1 in an offense that is decidedly short on receiving threats. I think he's currently rated several tiers too low, and while I'm not expecting it, I wouldn't be at all surprised to see him finish the season in the top 10 (sort of like Javon Walker his third season).
TE-
Jermaine Wiggins- Pop quiz: Who was Minnesota's leading receiver last year? Who was Minnesota's leading receiver the year before? The answer to both questions is Jermain Wiggins. Two years ago he was the #7 fantasy TE in the league. Last year, he fell to 17th, mostly due to the loss of 3 TDs and a little bit of ypc. Still, TDs are very volatile, while receptions tend to be more stable- and Wiggins gets a lot of receptions. He's now playing on a team that just suffered a big blow to its WR corps with the loss of KRob. He's playing with a QB known more for accuracy than arm strength in Brad Johnson. He's playing for an offensive coordinator famous for coordinating an offense that basically ran without any WRs until TO came to town, instead spreading a lot of receptions out among RBs and TEs. He's super-cheap, he's practically assured of outperforming his draft position, and he's a very sneaky late-round pick with a chance to crack the top 5 this season.
PK-
Jason Elam- Alright, the whole idea of a PK being a sleeper is rather ludicrous, but Jason Elam does NOT get the respect he deserves in fantasy circles. Consider this: Jason Elam has finished in the top 10 for fantasy kickers for 9 straight seasons. To put that into perspective... Vanderjagt and Stover are second, with three straight top-10 seasons. Jeff Reed, Rian Lindell, and Shayne Graham have two straight top-10 finishes. Outside of those 5, no one else has even back-to-back top-10 seasons. Elam's 9-straight top-10 finishes is ABSURD. He's pretty much a mortal lock to finish in the top 10, which should make him the no-brainer first kicker off the board.
Now, odds are that he won't be the #1 overall scorer at PK (he's only finished 1st overall once during that span, although he's been top-3 in 4 of the 9 years, and top-5 in 6 of the 9 years). Still, compare him to some of the other top kicker selections, such as Grammatica in '03 (finished 28th overall), Wilkins in '04 (finished 26th overall), and Vinatieri in '05 (19th overall).
Another underrated point is that Elam kicks in Denver. Everyone likes grabbing kickers in domes, but Denver's actually just as easy to kick in as a dome (if not easier), thanks to the extreme altitude. Denver has had a kicker finish in the top 10 in 14 of the last 16 seasons. Second place over that span goes to Tennessee, with 9 top-10 finishes in the last 16 years. Third place is a tie between Indy, Pitt, and Philly, with 8 such finishes apiece.
It's hard to argue that a kicker is underrated, but if any kicker is, it's Elam. The guy is money in the bank season after season, and yet he's never been drafted higher than 3rd (twice), and is currently the 5th kicker off the board. If 4 PKs have been selected, and there's no one else I am in love with, I will grab Elam without hesitation and not worry about PK for the rest of the season.
Defense-
New England Patriots- Remember a couple of years ago when the Patriots lost some ridiculous number of starting defensive players and still finished as one of the top fantasy defenses in the entire NFL? Apparently no one else does. This defense is going WAAAAY too late, considering it has a track record bested only by Tampa and Baltimore. New England's defense is the D/ST equivalent of the veteran player with a proven track record who everyone has written off after one bad season. They've done a lot more good than bad, and so I give them the benefit of the doubt.
Speaking of Tampa Bay, in one of my leagues, they haven't finished the season a single time this millennium as low as they're being drafted on average at the moment. Not a great "sleeper" because you'll still have to pay for them, but seriously undervalued.