Twig is supposed to be "a powerful yet lightweight tool for capturing and cultivating your ideas. You’ll be able to start using Twig right away, and yet you’ll have all the power you need to as your knowledge accumulates and your understanding grows." You can buy Twig preview 1.0.1 for the Macintosh. It will set you back $69.00.
It's supposed to work with TinderBox, the flagship application of Eastgate Systems. In other words, Tinderbox and Twig can share data "seamlessly" by reading each others files.
The main advantage of Twig as compared to TinderBox seems to be ease of use (which is certainly not one of the strengths of TinderBox as I recall).
I will not use it because I (unapologetically) use Windows 7. A Windows version of Tinderbox has been announced for years, but never materialized (and I doubt that I would use it now, anyway).
1. See also the reference to an introduction to Tinderbox. The fifth installment of this introduction has just been posted.