I recently (Nov.) started using Joplin, "An open source note taking and to-do application with synchronization capabilities" and I'm quite happy with it.
I've been a long time Evernote user (over 10 years). Although it was a bit rough at first (see A Plea to Evernote) it has worked well for me. Like Joplin, it meets my requirement for something that runs on Windows, Mac, and phone/tablet, and works off-line. (Joplin also runs on Linux.)
I'm pretty sure Evernote is an example of Conway's Law at work. Their versions have the same overall features, but there are enough small differences to be quite annoying when you're switching back and forth. You'd think someone at a high level would push for consistency. It's stupid stuff like one version puts a button at the right and another at the left. Then they came out with a new Mac version that was missing a bunch of features, and made yet another set of differences.
I've looked for alternatives in the past, but haven't found anything that matched my needs. I can't remember where I came across Joplin. I hadn't found it when I looked in the past. I wasn't specifically looking for an open source solution, but it's nice that Joplin is open source. It seems to have an active and growing community and user base.
One of the things I like about Joplin is that it's primarily Markdown. Most of my notes are plain text (see Taking Notes) but it's nice to have a little formatting at times. There is a new WYSIWYG editor, but previously it was the standard split screen edit & preview, or toggle back and forth. I mostly stay in the raw markdown mode.
One of the things I feared about switching was having to leave all my old notes behind in another program. But Joplin can import Evernote's export format. I haven't moved everything yet (there's a lot) but I transferred my Suneido notebook which is roughly 2000 notes. It took a little while to sync/upload and then sync/download on other devices but it worked well. There are a few formatting glitches, but that isn't surprising.
Interestingly, Joplin doesn't (yet) run its own sync servers. Instead you can use Nextcloud, Dropbox, OneDrive, WebDAV, or the file system. I already use Dropbox so that was the easiest for me. They are working on their own sync server software.
Since I got Joplin I have hardly touched Evernote. I use Joplin every day to keep notes on my work. If you're looking for a notes app it's worth checking out.
