Thinking about it, there were a couple more for me.

1) Penguin Eggs by Nic Jones. I first heard it at the age of twenty and it was one of those 'where have you been all my life?' moments. It blew apart everything I thought I know about singing and playing the guitar.

2) Seeing Vin Garbutt for the first time at Bridgnorth Folk Festival 2002. I can't ever remember a) being as moved by someone's songs and b) coming so perilously close to wetting myself with laughter. The man's a national treasure!