Do you need people to be able to see each others' calendars? Do you also need appointment details to be private when another employee looks at an employee's calendar?
I personally prefer Mozilla Sunbird but it's been discontinued. It lives on as an add-on called 'Lightning' for Mozilla Thunderbird, but I think that's simply because Mozilla has a hard-on for emulating Outlook (ie. e-mail and integrated calendar) when there's no real call for it.
With Sunbird / Lightning you can set up multiple calendars per user, network it, http/ftp it, and if you have the file system permissions correct then users can look at each others' calendars, but there's no private appointment support there.
The Microsoft solution is for you to buy Outlook + Exchange, which is not an insignificant investment. If you don't need the group calendaring support, then you can do without Exchange, which saves a fair bit and doesn't require IT support of another server-side bit of software.