I have a fairly liberal definition of "sleeper." I think it's any player that is being projected much lower than they should be. For me, DeAndre Hopkins is a sleeper. John Simon is a sleeper. Jordan Hill is a sleeper. The term sleeper implies a player that you probably haven't heard of before. But at the end of the day, the criminally under-rated player you've heard of is no less of a bargain.
Those sites tend to focus on players from division I schools that aren't being talked about a ton. Players like Korey Toomer last year. I liked the Toomer pick, but I never really thought of him as a sleeper. Actually, maybe for the Seahawks he was. But most late round picks that turn into great players do so because they went to the right team at the right time. To a coaching staff that knew how to get the most out of them. Both of our 7th rounders last year illustrated that perfectly.
"I believe there’s merit to the idea that once something in life becomes conventional, it’s no longer the safest path to success." -Matt Waldman
"If everyone's thinking alike, no one's thinking." -Bill Walsh