Key advantages include:
The Zimbra Collaboration Open Source Edition is available for your download free of charge. You are welcome to use, modify, and redistribute the Zimbra Collaboration Open Source Edition free of charge, provided you do so according to the terms of the appropriate License.
Why Network Edition?

We welcome all Open Source Edition users to join the Zimbra Community, and invite you to share contributions of bug-fixes, enhancements, or other modifications with Zimbra for consideration for inclusion in Zimbra.
Our principal reason for open sourcing Zimbra Collaboration is our hope to grow a thriving community around this technology. To that end, we cordially invite developers and systems administrators to join the newly inaugurated Zimbra Community in order to:
Help scrutinize our architecture and code in order to make Zimbra more reliable and more secure Shape future releases of Zimbra by helping us to prioritize the Zimbra engineering team's projects Provide value-add in the form of general purpose integrations, customizations, or extensions to Zimbra, which can (at the creator's discretion) be posted on the Zimbra community site (irrespective of any overlap with native Zimbra capabilities)
Integration may be of particular interest - Zimbra is an extensible collaboration platform that will provide programmatic and/or XML/web service bindings for:
We also welcome submitted code - ranging from bug fixes to substantial new features - for possible inclusion in the Zimbra Collaboration Open Source Edition. Submissions must be accompanied by an executed contribution agreement, which is more restrictive than the sharing model embodied within open source licenses. Questions? Please see the Zimbra Contributions FAQ.