The Roku Guide web site not only lists details on the regular channels, but keeps a list of hundreds of private channels:
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I like the iTunes channel for podcasts (Code ITPC). It’s nice to easily transition between Netflix and iTunes for things like Star Trek Mission Log Podcast, or Kumail Nanjiani’s The X-Files Files.
As if the Roku didn’t have enough channels already, there’s a decent selection of “private” channels you won’t find…