Leakymug is are joint podcast episodes put off by the hosts of The Leaky Cauldron's podcast Pottercast, and Mugglenet's official podcast Mugglecast. The episodes usually consists of one or more hosts of both podcasts participating together. They first came about in the early days of potter podcasting whereas Melissa Anelli appeared on a couple of episodes with the Mugglecast guys. Since then Leakymug's have been very popular amongst fans of both podcasts, and are put off now at Potter conferences or live events.
I attended a Leakymug while at the conference, Azkatraz.