Scrivener supports OPML, which is kind of like the RTF of structured outlining. It’s quite popular on the Mac, so you should find most mindmapping programs support it for both import and export, meaning quick and easy integration with Scrivener. I’m not too into strict mindmapping myself, so I cannot make a good recommendation, I can just list a few off of the top of my head. I know of Freemind (which is cross-platform), NovaMind (kind of expensive), and MindNode is a very basic one.
If you’re looking for something a bit more freeform, you might try our very own Scapple, which is currently in beta and available on the forums here. That’s not a mindmapper, but it is a visual environment where you can write here and there on a canvas and maybe link things up together, or maybe not. I’m liking the definition, “freeform text editor”. Because it just lets you type, it feels quite a bit different than programs using a strict hierarchical philosophy. Naturally it works well with Scrivener.
On the more complex end there is Tinderbox, which is more within the realm of what is referred to as a “concept map”, but it’s not strictly that either. It’s another freeform mapping program, but it is also programmable and one of the programs that, like Photoshop, really has no “bottom” to it. Tinderbox can open Scrivener projects directly, so it makes an ideal companion if you have the wallet, and the time it takes to learn it.
I use it on my MacBook and on my Nexus and they sync flawlessy via a nicely integrated DropBox system. It’s good to doodle with the touch-screen on the sofa or on the train and then have it on the Mac when I next get to the office.
OPML export to Scrivener (Curio, Omniwotsits etc) …