We have running at the moment eGroupWare, but we plan to migrate to SOGo (http://sogo.opengroupware.org) in the next two months (we had some annoying problems with eGW in the past). It has a really cool Webfrontend (looks like Thunderbird with Lightning) and has a really functional CalDAV-Interface which integrates perfectly into Thunderbird/Lightning. On the Website is a really good Install-Howto and it has even a yum repo.
Slightly OT, but just wondering if you are planning on running openLDAP on the SOGo Opengroupware installation, or whether or not you have an external LDAP server (CentOS DS or RHEL DS) that you are planning on using? Any experiences or gotchas that you have already encountered that might be useful? My company is planning on implementing either FDS or CentOS DS as an LDAP server, and I read the docs for SOGo, and they are using openLDAP on the same machine.
I develop on OpenGroupware, not SOGo, but both use SOPE's LDAP library/bindings. If the DSA supports LDAPv3 binds you shouldn't have any problems using it.
I'd recommend OpenLDAP any day, as it is far-and-away the faster and more feature-reach DSA. But I very much doubt it matters in regards to SOGo.