Google Calendar is a free online calendar application from Google. It allows users to create and manage events, share events with other users, and view calendar data from other sources. It is available on the web, as well as through mobile apps for Android and iOS. Google Calendar helps users stay organized by providing an easy way to view their upcoming events, tasks, and reminders. It also provides tools to help users manage their time and collaborate with others. With Google Calendar, users can create and edit events, send out invitations, and set reminders. Additionally, users can access their calendar from multiple devices, allowing them to view their events and reminders on the go. Google Calendar also allows users to share events with other users, which makes it easier for groups to coordinate their activities.
Open source, has widgets and is quite configurable.