I can vouch for Miro (formerly Democracy Player) - the content it starts you out with isn't that good and I had never previously bothered with it, but there's a decent amount of good stuff in the channel guide. Eg Nova, Frontline, several others. These aren't the full shows, but shorter segments. Because the seasons are fixed-length you get a bonanza at the beginning which runs out, but you can still use Miro as a podcast player (and queue up those Google.edus you've been meaning to watch), which is nice. Unfortunately Miro doesn't accept audio-only channels into their guide, but you can set up your own just fine.