Security Now is a IT security podcast presented by Steve Gibson and Leo Laporte. There is some controversy about Steve Gibson in the IT security community, but I'll discuss that as a separate issue once I get my IT blog running.
There are two main types of show: Listener Feedback and Themed podcasts, the split is 50/50 between these. Listener feedback includes a lot of Question and Answer content often related to the previous podcasts. The themed podcasts are usually about a few topics, such as the Phorm behaviour tracking software, Microsoft's Windows Steady State, Open ID and TruCrypt. There is a reasonable level of IT knowledge assumed which is appropriate for the content.
Leo Laporte acts as the facilitator for the show with Steve Gibson explaining issues and replying to the listener feedback that Leo reads out. This works well and Leo plays the role of an informed layman on the show.
There are a fair number of plugs for the companies that support the show, such as Astaro and Steve Gibson's company GRC with its product SpinRite. These are not too intrusive and reflect the reality of needing to support a podcast commercially unlike many of the others that I've reviewed.
At the GRC site there is an option to download a low bit rate version that is small and of a listenable quality. The high quality version can be downloaded from iTunes and the other usual sites such as Feedburner.
This is a useful podcast with plenty of sound advice amongst the product promotion. If you do dislike frequent mentions of sponsors or the presenters' own products you may not like it. I do listen regularly to this which shows that I do consider it worthwhile, but remember to look at other useful sources of security information like Bruce Schneier's blog.