Keepnote (Linux/Windows/Mac but awful. I love it on Linux/Windows) http://keepnote.org/
Quiver (Mac) http://happenapps.com/#quiver
Yojimbo (Mac/iOS) http://www.barebones.com/products/yojimbo/
NVAlt (Mac) http://brettterpstra.com/projects/nvalt/
OneNote for Windows (not open format, but can export into open format. Avoid saving notebooks to OneDrive, save them to local disk storage. OneNote for other platforms does not support local storage at all)
ResophNotes (Windows) Aims to be an NVAlt clone. I don't like it, but many others do. http://www.resoph.com/ResophNotes/Welcome.html