SideNotes is described as 'Clean and handy notes on your monitor side' and is an app in the Office & Productivity category. There are more than 100 alternatives to SideNotes for a variety of platforms, including Windows, Mac, Android, iPhone and Online / Web-based. The best alternative is Joplin, which is both free and Open Source. Other great apps like SideNotes are Standard Notes (Freemium, Open Source), Notion (Freemium), Google Keep (Free) and Microsoft OneNote (Freemium).
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.
With so many tools, it's hard to keep everyone on the same page. Notion is a new tool that unifies them all. It's like Google Docs, but everything is organized and discoverable by your team. Real collaboration in real-time.