Hollywood Saloon - The podcast is now defunct, but their archive is an absolute treasure trove for film fans.
Suffice it to say, I would say it's the best produced film podcast on the internet, and the closest you'll get to NPR quality programming (I'm talking careful editing, intercut music score, audio clips, director's commentary/interview segments). Check out something like their Art of Editing show, their Fury Road car chase show, or their Bond series.

I have a number I regularly listen to:

This Week in Science
The Skeptics Guide to the Universe
Junk Food Dinner
Cinefantastique- the Bad Film episodes
Monster Talk
The Human Bible
The Skeptical Viewer - this one seems to have died, but check out old episodes which go over old Ghost Hunters episodes and conclude that one of the two guys is doing the faking.

HDTV and Home Theater Podcast
Filmspotting SVU (Streaming Video Unit; I've kind of given up on Filmspotting itself)
Film Pigs
Judge John Hodgman
Lexicon Valley
Car Talk
The Doctor Who Podcast
Chinstroker vs Punter
NPR Science Friday
Radio Free Skaro

Yay!

The Film Threat Podcast

Slashfilm podcast

I just started listening to Niel deGrasse Tyson's StarTalk Radio, which is pretty awesome.

Vinyl Cafe Stories from CBC Radio

Owen, thanks for inspiring me to get off my ass and check out Hollywood Babble-On. I live in LA, so I'm one of the lucky ones who get to know Ralph from the radio show first.

Low key and funny. I like it.

Thanks to those who referred to "How Did This Get Made?" I've just listened to a few episodes and have been really enjoying the podcast.

I second the recommendation for Dan Carlin's Hardcore History, the Genghis Khan and Roman Republic series especially (if you're a history buff).