As much as I can't stand him and think he has been one of the most overrated fantasy guys the past few years, I will have to say Julio. The O's resigned him for a year yesterday and said that he will compete for the closer job in ST, meaning that Ryan does not officially have the job yet. If (when?) he loses out to Ryan, I could see him getting traded before the season starts to some team desperate for a closer. I am not saying the chance is any better than 50% he will have a job by the beginning of the season, but given that you could have him for a song this sounds like a good sleeper to me.
Well, he isn't definitely an unknown, but I like Guardado as a great value pick. The injury concern goes w/o saying, but if you have room for at least one of seattle's other guys, i think he is definitely worth the late round pick.
Actually I would argue that Ryan is the opposite of a sleeper. Everyone is talking as if he will step right into being a top notch closer, but he is completely unproven in that role and hasn't even been officially awarded the job yet. I would still like to have him on my team, but when people are valuing him like he is a sure shot.
Another sleeper is Tim Worrell, considering the injury problems that Wagner had last year.