I do Deep Sleeper posts early in the baseball season (my biggest hit this year was Casey McGahee, I think) so I figure I'll toss one out on the eve of the NFL season too.
I tend to play in deeper fantasy leagues, so the guys I look at as late-round lottery tickets could be guys you don't even have on your radar yet. I notice there's also a bit of overlap between my list and the one Peter already posted, so consider those guys twice as likely to break out!
QB: Joe Flacco - this is a pure hunch pick. Flacco did a little better than expected as a rookie, but he plays for a team that usually likes to run first, second and third if they can help it, and his main targets are either aging (Derrick Mason, Todd Heap), come into the season limping (Mark Clayton) or unproven (Demetrius Williams, Kelley Washington). Still, I have a good feeling about Flacco. His defense is capable of giving him a lot of short fields, Ray Rice has yet to prove he's an every-down back, and offensive coordinator Cam Cameron helped make Drew Brees and Philip Rivers stars back in San Diego. He still needs a go-to receiver to have a chance at being special, but even without one Flacco could be more than just bye-week filler.
RB: Bernard Scott - I haven't trusted Cedric Benson since the day he was drafted (and nearly burst into tears ranting about how sick he was of being compared to Ricky Williams in his first interview as a Bear), and I'm not about to start now. Scott has issues of his own, but he appears to have some talent and the Bengals have the potential to be a ground 'n' pound team built around an improving defense and their running game (rather than the unreliable Carson Palmer-Chad Ochocinco duo) so if Benson spits the bit again Scott is well-positioned to benefit.
RB: Felix Jones - he's essentially Reggie Bush without the hype. Jones is a legitimate threat to score every time he touches the ball, and the Cowboys have some holes in their wide receiving corps so they may elect to set up Jones for a lot of screens and short routes out in the flat. Staying healthy is key, but the potential is there for a real home run pick.
WR: Earl Bennett - yup, I'm still on the Bennett bandwagon even though he didn't have a huge preseason. The Bears did not trade for Jay Cutler just to have him hand off to Matt Forte, and while Devin Hester is thought to be Cutler's top target, Hester still doesn't run the sharpest routes or have a lot of experience as a wideout. Cutler does have history at Vanderbilt with Bennett though, and they worked to rekindle their chemistry this offseason. I think Bennett's a better pick this season than either Percy Harvin or Jeremy Maclin.
WR: Josh Cribbs - he doesn't get the attention Hester does, but Cribbs is following the same kind of career path (lethal return man converted into wide receiver) with the added bonus of also being used in the Wildcat on the rare occasions the Browns bust out the formation. He also plays for a team that will probably be airing it out a lot because they'll be playing from behind more often than not, so even though he's not Cleveland's #1 WR (that spot still belongs to Braylon Edwards) there should be plenty of targets for Cribbs if he shows he can handle them.
WR: Limas Sweed - Hines Ward barely sniffs six feet (and is 33 years old), and he's still taller than Santonio Holmes. Sweed, meanwhile, is six-five and looked much, much better this preseason than he did as a rookie. Ben Roethlisberger's been dying to have a red zone target like Sweed he can throw to ever since Plaxico Burress left town, and with neither Willie Parker nor Rashard Mendenhall looking like RBs who can be relied on to punch it in near the goal line Roethlisberger could be putting it up from in close a lot.