Been meaning to do this for a while, but this is a list of the stuff I *try* to catch each episode, or stuff I've enjoyed that's on hiatus but worth subscribing to either for the back catalogue, or a possible new series.
The categories are a bit arbitrary (The Amateur Scientist would fit in Science and Skepticsm too, and Collings and Herrin might possibly fit in news) but do represent what I listen to them for.
Most of the links below go to title pages rather than the RSS itself - I thought this better, as a description for each 'cast would make this a very long post. I subscribe via RSS - I think most of the 'casts have an iTunes feed too, but I can't say for sure as I don't use iTunes.
All the feeds are free, and some have a "tip jar" system for you to donate to the creators if you like the content - others have subscriptions, and some will encourage you to pick up the authors' work or merchandise in other, paid media if you like what they're doing.
Collins and Herring (6 Music Show)
Friday Night Comedy (Radio 4)
Richard Herring's As It Occurs To Me (NSFW - lots of bad language & adult themes)
The Amateur Scientist (NSFW - occasional bad language & adult themes)
The Collings and Herrin Podcast (NSFW - lots of bad language & adult themes)
You Look Nice Today (NSFW - occasional bad language & adult themes)
Carrément Vélo (in French)
Other Factual (Documentaries, History &c)
Music & Film
News & Current Affairs
Newspod (Round up of the news of the day from the BBC)
Global News (Round up of the news of the day from the BBC World Service)
Science and Skepticsm
Short Stories and Fiction
Cast Macabre (Short form horror)
Clarkesworld (Short form science fiction)
CrimeWAV (Short form crime and noir on the hardboiled edge of the genre)
Escape Pod (Short science fiction)
Podcastle (Short form fantasy)
Pseudopod (Short form horror)
Scott Sigler Audiobooks (Novel length serialised horror and science fiction, and Scott's reader feedback and interviews elsewhere. Definitely not for kids. The current serial is "Ancestor", but Scott's back catalogue is available too, including fan favourites like "Contagious" and "Earthcore")
Toothless (Novel length serialised horror - an interesting twist on the zombie genre so far - not for kids).
Underwood and Flinch (Mike Bennett's novel length serial tale of a vampire and his reluctant servant in modern day Spain - again, not for kids)
(Also worthy of mention is the MIGHTY podiobooks.com, which has more serialised audio fiction than you can shake several sticks at, and an excellent personalised subscription system).
It's worth noting that the biases of the Internet mean that much of this is horror, fantasy and science fiction - and lots of it is written specifically to be somewhat disturbing (it's horror, after all!). As I recall, Clarkesworld, Escape Pod & Podcastle will give a guidance rating with stories, so should be safe for those of a nervous disposition (as long as you pay attention to it).