Humble Daisy's ProfCast is a well-respected tool for turning PowerPoint and Keynote slideshows into podcasts, among many other features (including the ability to record live presentations). The software was recently updated to version 2.3.0 and if you haven't been motivated to try it before, it is definitely worth a look now.
Humble Daisy is currently offering the software for 75% off its retail price from today until January 24 to celebrate the Mac's 25th birthday. ProfCast normally retails for $59.95USD, and education discounts are available, but the sale brings the price down to $14.99. ProfCast requires OS X 10.4 and later.
And for the confused, myself included, ProfCast's Web site does say that the discount is for 25% off rather than 75%. However, by clicking on the coupon for a single-seat license, you can see that the price really is $14.99.