Having an extremely long drive to work every morning followed by an extremely boring day, I listen to far more than anyone should.
The best place to start is here:
http://www.digg.com/podcasts
Some of my favourites include:
- The Sceptics Guide to the Universe
- PC Gamer (UK & US)
- TWiT
- Point of Inquiry
- 12 Byzantine Rulers
- GFW Radio
- Cranky Geeks
- ExtremeTech Podcast
Also if you can find it I highly recommend The Ricky Gervais Show (series 1-3). You'll have to find a torrent if you want to listen to it for free or if you already have an Audible account you can get it from there.