Take a look at the Calendar compatibility Matrix to choose what is the right for you.
We run a web-based groupware system (Sogo) that allows calendar management and collaboration (sharing/editing) with other D-PHYS users. It provides a personal calendar for every user (by default) and group calendars (contact us). It also syncs calendar entries to a wide range of desktop and mobile clients.
We also host events.phys.ethz.ch, a web-based event announcement system. Apart from the displays around campus, RSS and PDF downloads, the events are also provided as iCal subscriptions - see the link below.
If you need to coordinate meeting times, have a look at ETH's
doodle. This time management tool allows to poll collaborators to determine the best time and date to meet.
The Informatikdienste of ETH use Microsoft's Exchange for email and calendar.
If you find yourself receiving a lot of Exchange meeting requests from other people at ETH, you might be interested in the solution we have found for our groupware: use Thunderbird + the Lightning plugin and sync to eGroupware as described here. If you then read the invitation email in Thunderbird, it detects the event and allows you to accept the invitation.