Itsycal is described as 'tiny menu bar calendar. If you want, it will display your events as a companion to the Mac Calendar app' and is a Calendar in the Office & Productivity category. There are more than 10 alternatives to Itsycal for a variety of platforms, including Mac, iPhone, iPad, Linux and Apple Watch. The best alternative is Fantastical. It's not free, so if you're looking for a free alternative, you could try MeetingBar or Kalendar. Other great apps like Itsycal are GNOME Calendar (Free, Open Source), Calendar Lock PEA (Free, Open Source), BusyCal (Paid) and Calendar 366 (Paid).
Fantastical is a flexible calendar with natural language event creation, a beautiful calendar and event list, support for CalDAV (including Google Calendar delegates) automatic alarms, and instant searching.
Kalendar is a Kirigami-based calendar application that uses Akonadi. It lets you add, edit and delete events from local and remote accounts of your choice, while keeping changes syncronised across your Plasma desktop or phone.
GNOME Calendar is a simple and beautiful calendar application designed to perfectly fit the GNOME desktop. By reusing the components which the GNOME desktop is build on, Calendar nicely integrates with the GNOME ecosystem.
InstaCal is an affordable, yet powerful calendar app that puts all your events right at your fingertips, always available in your Mac's menu bar. InstaCal is quick and convenient, and can be opened at any time with a configurable keyboard shortcut.