FeedDemon, the popular RSS reader, started life as a commercial product.

Then it became ad-supported.

And now the latest version, FeedDemon 4, makes use of both approaches: installing the program will get you a free, Lite version, but paying $19.95 for a licence will get you the full-strength FeedDemon 4 Pro, with a stack of extra features (and no ads at all).

There’s “My Stream”, for instance, which checks frequently-updated status feeds (Twitter, Facebook, Gmail and so on) more often than other feeds, displaying new articles in a sidebar just as soon as they appear.

You get four Newspaper Styles, each of which provides their own way of displaying your articles (one line summary, 6 lines for more detail, etc). You’re able to switch styles at a click, or assign specific styles to individual feeds so they’re always displayed as you’d like.

If you find your feeds are overwhelming you with junk you don’t really need, then applying Content Filters may help. You can ask FeedDemon to look for particular keywords in the article’s title, description, category or author fields, and the program will then only display items that match your requirements.

And other interesting enhancements include prefetching (download links and images in unread articles so you can view then when you’re offline), FeedStation support (download multimedia files and sync them with your iPod or other media player), and the ability to subscribe to authenticated feeds, including those using Twitter OAuth.

FeedDemon Lite users will see improvements, too: author Nick Bradbury says the program is “significantly faster than previous versions”, and “the interface has been streamlined and simplified to make it easier to use”. But the real power is clearly in FeedDemon Pro, where the new My Stream feature in particular will help you keep track of all your social networking feeds.