1) Starstuff:
http://www.abc.net.au/science/podcast/starstuff.xml
Excellent, excellent, excellent. Nothing like it elsewhere, we're so lucky to still have it going as a podcast even though the ABC took the show off air
2) AstronomyCast (previously mentioned)
3) Cranky Geeks:
http://www.crankygeeks.com/
This appeals to the software developer in me. It's not astronomy related at all. A bunch of cranky hosts talking about current news in the IT world. I find it strangely entertaining and informative
... I almost put
365 Days of Astronomy in the top 3, but I find many of them boring and they're of a lower production quality, so it doesn't rate above the others for me.
Roger.