Many Mac users seek an alternative to OneNote if they've just switched from Windows and Growly Notes is one of the strongest contenders.
Growly Notes lets you capture just about anything all in one place. It's a great tool for organizing research projects, trips, to-do lists, or journals. In addition, its equally competent at creating scrapbooks of random images, web links and video clips.
Like OneNote, the great strength of Growly Notes is that it can contain almost anything, including formatted text, images, movies, audio clips, PDF files, tables, lists, web and file links, and drawings you create in Notes. You can place things anywhere so you don't have to worry about inflexible templates or rigid rules. You can separate ideas and brainstorming sessions into tabs which you can navigate very easily.
It's impressive that Growly Notes offers for free many of the features of a highly polished application like Curio. Growly comes amazingly close to OneNote in many aspects with the bonus of a slick OS X interface of course. However, some of the more advanced features of OneNote - such as integration with OneNote web app and syncing with Outlook aren't possible in Growly Notes.
Growly Notes is an excellent free Mac alternative to OneNote and is perfect for mind-mapping, note-taking and brainstorming.