Check out the Sussex Folk Guide.

The Willows is friendly, packed out on Guest nights, but I personally find the open stage nights musically dull - though still friendly.

The two Lewes clubs - the Saturday club at the Elephant & Castle, and the Thursday Club at the Royal Oak - are both excellent and consistently good. Also good clubs at Horsham and Seaford. And more sessions than you can shake a stick at, if you enjoy such things.