The Signal- This is the reason I started listening to podcasts in the first place. It's the podcast of the Firefly 'verse! It's extremely well done, and has a lot of cred with the cast and crew of Firefly. "Season One" of The Signal focused on promoting Serenity. "Season Two" (which just started today) focusses on getting a sequel made. As a side note, I actually learned some Chinese from this podcast. :-)
Firefly Talk- This is the other podcast of the Firefly 'verse! ;-) The crews of Firefly Talk and The Signal have about a 50% overlap, so the quality level is pretty much identical. Surprisingly, there's almost no overlap in material between the two shows. Firefly Talk tends to focus more on the fan experience. It's The Signal's little brother, but is just as compelling.
Polyamory Weekly- cunningminx is rapidly becoming a quasi-official voice of the poly community. Her show is fun, informative, and well-balanced. She also has a drop-dead sexy voice. :-)
Ask A Ninja- I've written about this before. It's the only video podcast to which I subscribe, and it's a riot! Got a question? The ninja's got answers! Of course, it'll probably cost you your life...
Skepticality- Science and Skeptic Thought! This is a breath of rational fresh air, debunking urban legends, quackery, and common historical misconceptions in a lighthearted, fun format.
Battlestar Galactica- This is executive producer Ronald D, Moore's official commentary for each episode. These podcasts are the same commentary that appears on the DVDs, so each one has the same running time as the corresponding episode. He delivers a lot of insight into what happens "behind the scenes". Interesting side note- his commentary for the most recent episode (#214- "Black Market") is almost entirely a litany about how bad the episode was and what went wrong with its production.
The Combat Information Center- This is a fan podcast that compliments the official podcast perfectly. Where the Sci-Fi podcast is behind the scenes, Alan Light's podcast is like talking about the show with a fellow fan. He's sharp and often has interesting observations. Interesting side note- his commentary for the most recent episode (#214- "Black Market") strongly focused on how much he loved the episode and pretty much everything about it. Quite a contrast. :-)
Radio Verda- My study of Esperanto has largely fallen to the wayside, but I still have a strong interest in learning it. Part of the problem I faced was a lack of opportunity to hear la lingvon being spoken by someone who is fluent and well-practiced. This fixes that. Each installment of Radio Verda is short but interesting. I enjoy both using it as background noise to grow more accustomed to the sounds of the language and listening closely to it to try to suss out what's being said. Most of the time I only pick out a word or phrase here or there, but on those occasions when I can work out most of a conversation it's like a little linguistic treat.
By the way, in case you don't know, they're all completely free. You can download the individual files from the linked site, or subscribe using your podcast catcher of choice. Personally I use iTunes (I know, you're shocked). You can go to the iTunes Music Store, search for a podcast by name, and subscribe with a single click (yes, still free).