For Mac only:
Since being turned on to the Omni Group's "Omni line" of productivity applications I've been hooked and won't look back. I do sound design, preset libraries and beta testing for a plug-in company that is essentially a one man operation. My colleague asked one day if I had any ideas for a plug-in concept and boy did I. This turned into a situation where I'm doing the design and he's doing the programming. For the design and getting the ideas out of my head he suggested OmniGraffle.
He initially showed it do me because It's really good for dumping data and ideas out of your hear and into the computer, then it's ridiculously simple to manage, modify, arrange, associate, link and everything else you might want to do with a random documented thought ad-infinitum. I started using it to make lists of features, ideas and random and then I set into doing block diagrams in it. An example would be lock diagrams of the signal flow and dsp stages of the plug-in.
After that experience I checked out their other products and they're all equally as great and compliment each other amazingly well. The other one's I use are OmniFocus, OmniOutliner and OmniPlan, as they all do something different, equally as important and simply compliment each other. Similar workflows in all of them. They also have ipad and iphone versions so you can work on the go and they all stay synchronized (with backups is set in preferences. Overall I'd say I'm really happy with them.
It's basically a software suite consisting of standalone programs that have been painstakingly considered and perfectly executed (considering their intent and how hard that is to usually achieve). They're done so well that if you using their software the way it's intended will make you more organized, efficient and minimize (and prevent doubling of) efforts on tasks or other things and so on. Effectively helping you to help yourself and streamline and effortlessly manage your life. Check out some of the videos they have on their site. Once they start showing you stuff the software practically sells itself.