Basically a clawhammer style of fingerpicking but with a lot of odd chords + string-bending; also some unexpected changes of time signature. You could try listening to John Fahey and Jimmy Page, or go back to the roots with the Reverend Gary Davis and Blind Boy Fuller.
Bert Jansch is one of my heroes; just got tickets to see him in a couple of weeks! Musically, he's folk/jazz/blues. His voice isn't to everybody's taste, but his technique is amazing.
Similar people: you could try his band Pentangle, or band mate John Renbourn. Then you have the likes of Richard Thompson, Nick Drake...I could go on and on!