I think most of the people here used ReminderFox for fetching date entries to be informed by Thunderbird.
I haven't checked a FULL ReminderFox <--> Zimbra Synchronisation with all possible options yet..i just had no time ;-)
ReminderFox can also manage taks, but Zimbra doesn't. Right now RmF is only a crutch for me, waiting for the long requested requirement in Zimbra:
- Adjust reminder alarms for EACH appointment !
- Send reminder alarms per email to the user
On these really simple functions Zimbra fails , which is more than a pitty, if you compare these to all the other "cool" Web 2.0 functions Zimbra can handle.