Malcolm: Bright Phoebus, Royal Traditions, Kelham Island singing sessions, the uni ceilidh society, Pecsaetan Morris, Sheffield City Morris, Sheffield Giants, Boggarts Breakfast, Glorystrokes/Hekety and their respective members' bands and projects, Trinculo, Hield/Bodens, Simpsons, Kerr/Fagans, Sheffield Carols...these all spring immediately to mind. I'm sure others could add to the list of what makes Sheffield a folk hub at the moment.