Made by the same company that makes Scrivener, Scapple is brainstorming and notetaking software that allows you to take non-linear notes, connect ideas as you work through them, print your notes, and even move them into Scrivener.
Scapple is available for both Mac and PC. It has a one-time $18 standard license fee ($14.40 for students and academics with an affiliation). There is also a 30-day free trial period (and that's 30 days of USE, not just 30 days total) that gives you plenty of time to play with it and see if it words for you before you buy.
The FAQ section for Scapple notes that minor updates will be available for free, but if they make major updates down the road, they may require users to pay to upgrade. With Scrivener, though, Literature and Latte (the company behind both) has shown that it works to provide as much value as possible and rarely makes the sort of major updates that require payment.