I would like some URLs to free collaboration suites. It is important for them to be as free as possible, because they will be used for a non-profit purpose. I am afraid I am a newbie in this area, so my search in the Internet is a bit painful. If somebody knows of any suite with what is considered "most expected features", I would greatly appreciate it. Right now I do not even know what those features are supposed to be, so I will also appreciate short descriptions of those if anybody has the time to list some that they consider most important.
Thanks for the clarification! I'm sorry my first post wasn't right on target for you. :(
Zimbra certainly is the heavyweight in this area. Possible limitations on the free version for you might be:
-Online backups/restore (solve using rsync or similar)
-Clustering (you can use Heartbeat on linux servers though)
-Rebranding (might not be an issue for you, or you can use an in-house proxy service to swap image files on the fly)
I don't know if any of the above items are deal-breakers for you, though. Overall Zimbra is probably poised to be the most actively-developed suite in the field, as Yahoo! is behind them. If a large user-base and active development are key features, Zimbra is probably your best choice.
Since you're looking for lowest possible cost solutions, there are a few other suites out there you might want to consider (if only to get a few more ideas):
If you want ideas about features and other uses for this type of software, read the individual items under the "Applications" section. Each area is thoroughly detailed and might inspire you.
Also, this package is available in a Greek language localization (it appears there's also a consultant linked through their website as well).
Very flexible, but will require more advanced administration skills to set up and maintain.
This suite, like Zimbra, has paid and free versions available. I'm assuming you want the free version, so be sure to focus on the "Community Edition" (the site can be a little confusing). You can also download the community version through sourceforge.
Last but not least! Another good candidate for you. Robust feature set, although a The Greek language localization isn't complete, however the major administration functions have already been translated. This is probably the second-most popular package in use (after Zimbra).
Although this isn't collaboration-related it might be interest to you:
There's a great set of tools for education available in a package called Moodle. (http://moodle.org/) I highly recommend anyone who is looking to make every dollar of their education (or training!) budgets check it out, it compares really nicely against commercial offerings like BlackBoard, etc.
I hope that helps a little, just let me know if you're looking for more information on any of the above!