• QMS Qmail and MailScanner intergration
• Citadel Linux Open Source Groupware Server
• Istanbul Free GPL Linux Screen-cast software for gnome
• Xvidcap Linux Open source Screen-cast software for gnome
• Nvu Linux Open Source Web Authoring program. Rivals FP and Dreamweaver
• scribus Linux Open Source Desktop Publishing, like Pagemaker from Adobe
• Nvu Linux Web Authoring program. Rivals FP and Dreamweaver
• QCad Open Source 2D CAD for Linux
• Bhpos Open Source Linux POS software

These products may not be Open Source

• Silk Systems Complete Linux based Business Management Systems POS. Stock control, Invoicing and more...
• MegaTech Complete Linux based Business Management Systems
• Merchant Tech Linux based Comprehensive Retail System Solutions
• ABS Linux based POS systems
• Appgen Linux based Accounting software

Linux is a free and Open Source Computer Operating System, one that powers many of the worlds corporate and Internet Servers, used from a 1 man office through to large mission critical operations like, ISP's, Telecommunications companies, NASA, and powers the best known named network on the 'Net, Google

The Linux kernel was first developed by Finish University student Linus Torvalds back in 1991, its maturity, performance, reliability and stability is second to none, and is a clear winner over many of the so called big named software companies.

The beauty of it is it's also a fantastic desktop, there is no reason to pay the likes of Micro$oft hundreds of dollars when better alternatives exist for FREE! And no restrictive M$ EULA's, share Linux as best and many times as you want, LEGALLY! Read more here.
There are many distributions around, but the better known ones for servers would be Slackware or CentOS, and for desktops, clearly Ubuntu has taken the world by storm.