Windows only: Free application Skype Call Recorder creates MP3s of any Skype conversation with a couple of simple clicks.
Given Skype's popularity, it's not surprising that there are plenty of alternatives in this space, but Skype Call Recorder is probably the most simple option we've seen. Install the app, run it, and hit record whenever you want to capture a conversation.
The app offers a few simple recording settings (mono or stereo and custom bitrates), but there's very little else to fiddle with—which is a strength. Skype Call Recorder is a free download, requires .NET 2.0.