In addition to Google Calendar (
www.google.com/calendar
 <http://www.google.com/calendar>) you might also look at
 
www.spongecell.com <http://www.spongecell.com> and 
www.kiko.com
 <http://www.kiko.com>
 I've used all three to some extent, as I did significant research over
 a couple of months trying to find a free, no ads, no spam solution for
 a bible study group I lead at my church.   I needed a web-based
 calendar solution -- but one that is either low-cost or free, and has
 no spam or ads......   You might think this would be an easy search
 with lots of solutions.   And, yes, there are a lot of webcalendars
 out in the market.
 However, what really narrowed the list of solutions down is the next
 requirement:    The web-calendar needs to be able send out
 invites/RSVP's to an event that was added to the calendar and then
 handle the responses (yes, no, maybe) as well as comments from the
 guests.    As if you are holding an event, you want to be able to
 track who is coming and any details about them (what food they are
 bringing to the event, etc...) in the comments section.
 These are the only three solutions I found, and all three work
 reasonably well.   I'm hoping that the XWiki team could add some (or
 all) of these functions to XWiki or find a way to integrate one or
 more of these solutions into an XWiki Site.
 The current calendar in XWiki is really not usable in its current
 incarnation (in my opinion) as it lacks important functionality.
 (but everything else about XWiki is really cool).
 David K. Whisler
 
Thanks, David. Your experience is of major importance for us!
Please, what solution have you finally adopted?
I will sign up for both Spongcell and Kiki trying to evaluate the pros
and contras of each option. I am quite happy so far by using Google
Calendar: it is rather easy to integrate it in Thunderbird. What I have
not clear at all is if it is possible to set up Google Calendar to
automatically send email reminders to a given event attendees. Do you
know if this is possible by using any of the other solutions?
I am afraid this thread is going of topic: we are not speaking about
XWiki :-) Even though it could be useful to keep this thread updated to
help further developments in XWiki Calendar.
All the best,
Ricardo
--
Ricardo Rodríguez
Your XEN ICT Team