Notenik is described as 'note-taking app for the Mac that provides unusual levels of flexibility in terms of the types of information stored, the ways that notes can be organized, and the formatting available for output' and is a Note-taking App in the Office & Productivity category. There are more than 25 alternatives to Notenik for a variety of platforms, including Mac, Windows, Android, Online / Web-based and iPhone. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Microsoft OneNote. Other great apps like Notenik are Evernote (Freemium), CherryTree (Free, Open Source), TreeSheets (Free, Open Source) and Omni Notes (Free, Open Source).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.
NixNote, formerly Nevernote, is an open source client for Evernote . It is written in Java so it will also run on Windows & OS-X, but the primary focus has been to try and get a usable environment for Linux.