Powerful Software
Freely Available* (Used Daily)
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Univention
Boasting the ability to replace your Windows Server AD infrastructure with Linux, Univention and their Corporate Server may be able to do just that with samba4, ldap, kvm, docker and an easy to use management interface.
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ScrollOut
Arguably the most powerful and easy to use spam filtering relay for the money. Particularly since it's free. With features too numerous to list, ScrollOut is the spam blocking machine every mail server should sit behind.
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Nextcloud
Forked from OwnCloud, this open source project is an out-of-the-box solution to file sharing and sync needs. With so many add-ons available, NextCloud can be turned into your own private office online.
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Docker
Containerization is the hot trend in IT and Docker is at the forefront. Their solution makes deploying your service a one-liner and scaling it nearly as easy.
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Observium
Keep track of your systems via SNMP and get important alerts with this free monitoring tool. Monitor workstations, servers and applications 24/7.
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OpenNebula
When you're ready to build your next private cloud, better take a good long look at OpenNebula. A VM management and orchestration engine designed to make your deployment life a whole lot easier.
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FusionPBX
This IPPBX fuses an easy to use web-based management gui with the FreeSwitch Software Defined Telecom Stack to create a powerful VoIP based communications solution for any business.
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Flectra
ERP systems are vital to businesses interested in agile management and rapid growth. Forked, quite contentiously, from Odoo (formerly OpenERP), Flectra boasts the same modular framework providing a flexible platform to build any business and keep it running.
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pfSense
The Internet is a dangerous place filled with bots and worms and probes and the attackers behind them. You need protection and the pfSense firewall has layers of it.
* Most, if not all, of the products listed above are freely available to download and use. Free versions may have limitations and/or missing features when compared to their paid version. Use of any of the listed software is conditioned upon their respective licenses.