Dynasty league (12 teams):
Prince Fielder (5th round - 55th pick) - not what I'd classify as a pure steal, as Fielder was highly touted to break out but power production akin to Ryan Howard is a brilliant boost with respect to the draft position.
Gary Sheffield (8th round - 90th pick) - was surprised he fell quite a good bit, but still thought the upside at that round was worth it.
Erik Bedard (9th round - 103rd pick) - not quite a huge steal, seeing how in some drafts, he's fallen to the 12th-13th round range, but a top 5 SP in the 9th is a terrific bargain nonetheless.
Kelvim Escobar (15th round - 175th pick)
Brad Penny (17th round - 199th pick)
Jorge Posada (18th round - 210th pick)
Orlando Cabrera (25th round - 295th pick) - was a dope to drop him in early April, but had Rollins to cover at SS anyway
Corey Hart - FA
Tom Gorzelanny - FA
Not a ton of steals in my other leagues besides my H2HWS USA league. Drafting M-Cab and Hanley in the 2nd and 3rd rounds respectively has paid tremendous dividends, but my best pick had to be Russell Martin in the 19th round and the best FA pickup I made was Brandon Phillips.
Oh, and in a Yahoo Winners league, I got Alex Rios in the 10th round.