nvALT - A fork of the original Notational Velocity with some additional features and interface modifications
productboard - Beautiful and powerful product management.
Notational Velocity - Notational Velocity: modeless, mouseless Mac OS X note-taking application
ProdPad - ProdPad helps your team gather ideas, surface the best ones and turn them into product specs, and then put it all on a product roadmap.
Zim Wiki - Zim is a graphical text editor used to maintain a collection of wiki pages. Each page can contain links to other pages, simple formatting and images.
ProductPlan - Cloud-based product roadmap software solution