Here's my list of players that you can get later than normal and might produce top notch numbers:
Matt Morris: Low wins total will probably push him a few round back from his 3rd round draft slot this year. Should be fine with an offseason to recover and could be a 20 game winner.
Roy Oswalt: More of a health concern than Morris, but people might let him slip way too far. If you can get him as a #2 or #3 starter, could be huge.
Josh Beckett: Won't be a total unknown, but with only 9 wins, might fall under the radar for too long. Has come on very strong, and may finally be ready to dominate on a Prior-like level. A steal if you can get him 5th round or later
Jose Contreras: Yanks will probably be looking at him to step up as their #2 or 3 starter with Clemens, and possibly Pettitte and Wells leaving. Has been great as a starter, and should be an excellent #4 or 5 starter next year for a fantasy team
Javier Vazquez: He's not going to slip too far, definitely not beyond the 4th round, but even there he could be a steal, since it's likely he is going to a new team in the offseason. If he lands with the Yankees, Red Sox, Twins, or any team with a better offense, he should get to around 18 wins, and his strikeout totals would make him a fantasy ace-calibre pitcher that you might be able to get more cheaply than most
Jeff Weaver: Talent seems to still be there, and if he can recover or is traded to a new team where he has a fresh start, could return to 2001-2002 form. Could be a steal as a late round flyer.
Shawn Green: Will probably slip way back in drafts, but he is having shoulder surgery, and that could be the reason his power is so down this year. Could return to 40+ HR level next year.
Brian Giles: Went in the 2nd or 3rd round this year, and might slip back a couple rounds because of subpar season. Next year, he will have a great lineup around him, with Nevin, Klesko, and with the Padres probably signing another big name, will have more talent around him then he ever had in Pittsburgh. Primed for a career year in 2004.
Ryan Klesko, Phil Nevin: Ditto for both of them. Both should be at 100% from injuries, and Padres lineup could be a fantasy goldmine.
Vlad Guererro: Because of injuries, and concern about lingering back complications, he probably will slip to the late 1st round next year and maybe into the second round. Because he likely will be signed by a new team, and a team that probably will give him a much better supporting cast, the injury risk is well worth it if you have a late 1st round pick. Like Giles, I think he could have a monster year next year.