Evan Williams, Blogger creator and now CEO of Odeo, has recently put together a podcast directory roundup.
Sadly, Evan uses the virtually useless Alexa rankings to rank the sites, but notwithstanding the order, he's got some good comments on each one. There are some comments spread about the blogosphere that the list is incomplete and some of them are probably warranted. I think it's unreasonable to ask anyone to enumerate anything in the podcasting space accurately, however. There are so many yo-yo's popping up and down that the space looks different every ten minutes.
I also think that Evan has blurred the lines between directories and networks, but that may be a reflection of real life rather than a shortcoming on Evan's part. The 'traditional' network model doesn't seem to be paying off very well.