By today's standard, sleepers aren't necessarily guys that are drafted in the late rounds who have little value at the time, I would consider them players who will significantly outperform their value or draft position.
Every year there are guys you can get in the mid rounds that end up being top fantasy options. For example, in a 10 team league last season I used back-to-back seventh and eighth round picks on Hakeem Nicks and Arian Foster. The bad news is that I took Shonn Greene in the early third round.
I didn't do the best job I could have in the first three rounds, but my mid round drafting ended up winning me that particular league. This year there are a few guys I'm targeting in those important middle rounds that have a chance to put up big numbers and ultimately win you your league.