Re: 800lb Gorilla over there sitting beside the Elephant...
Kontact and K-Mail are capable of connecting to groupware servers for collaboration.
I tend to use it to connect to my Google account and sync my calendars and email.
KDE Kontact supports various groupware servers. When using these servers your workgroup has access to features like shared email folders, group task lists, calendar sharing, central address books and meeting scheduling.
I don't know how that compares to Outlook as I don't use Outlook, but I do use Kontact and K-Mail all the time and it's more than adequate for my needs.