I have experience with OpenGroupWare it can be a bit tricky for Mac/Windows clients but, not all that hard to implement. If you are looking for easier and robust deployments and still want to keep it free. You should take a look at either:
Zimbra – Community Edition (Open-Source)
However, OGW and Egroupware are solid products as well took a bit of tweaking and tinkering, and few nights of coffee and Jolt Cola to get Opengroupware where I wanted to be bt, in the past year I have implemented the above two mentioned products Zimbra, and OX recently for other sites.