TextStitch is a popular free plug-in; it helps you with threading or separating text frames in InDesign CS, CS2, CS3, CS4, CS5 – all this without disturbing the look and feel of the page. There are three main functions: auto-stitch, quick stitch and auto-unstitch.
TextStitch 1.0.7 now also handles text-on-a-path alongside with normal text frames.
It recently got a new boost; it turns out to be helpful for ePub work. Read all about it in Keith Gilbert’s article on InDesignSecrets: