Word of mouth is probably the number one way to hear about new podcasts. But podcast directories are still handy for searching for podcasts on specific topics. There's also lots of good stuff on feeds that have gone dark, no new episodes but the feeds are still maintained. They also a great resource for those who are new to podcasts and don't know where to start. They have not listened to many shows, or not to the right shows to get the word of mouth info on good shows yet.