Sylar's original ability is the power to determine exactly how things work. Sylar appears to have used his innate ability to discover how to take the abilities of other evolved humans. The exact mechanism by which Sylar gains these abilities is not at present known; however, he believes that Chandra Suresh is correct that the abilities are based in the brain, and several of Sylar's victims have had their skulls opened and brains removed.
During their conversation, Sylar and Mr. Bennet confirmed that Sylar has multiple abilities (Fallout); however, apart from telekinesis, the Company was unable to identify what these powers were (Godsend). Based on Mohinder's statement to the authorities (Fallout) that at least six people from his father's list have been murdered by Sylar (prior to Zane Taylor), it's likely that Sylar has at least ten acquired abilities: the six Mohinder mentions, plus those of Zane, Dale, Isaac, and Ted.
Sylar attempted to take an eleventh ability from Candice Wilmer, but he was unable to use it because he was affected by the Shanti virus; it was confirmed in an interview that Sylar did not acquire the ability and that he won't be able to use it even after curing himself.
It is unknown if Sylar took Alejandro's ability since Sylar was interrupted by Maya when she knocked on the door. It is unclear if he went back for it or just left him there.
Sylar has demonstrated the ability to use at least two of his acquired abilities at the same time. (The Hard Part)
Telekinesis is Sylar's most commonly observed acquired ability.