Windows: If you're a fan of widgets and other desktop embedded sources of information, Feedling can park RSS feeds on your desktop in a transparent overlay.
Feedling isn't a feature rich RSS reader but it does what it sets out to do: put RSS feeds on your desktop. Our screenshot here shows a single RSS feed but you can stack multiple feeds together to turn an entire desktop into a feed reader. You can configure the font, font size and text colour independently for each feed, a convenient way to make your more important feeds stand out.
Feedling is freeware, Windows only, and requires Windows .NET 3.5+. Have a favourite widget, RSS-centred or otherwise? Let's hear about it in the comments.