From wikipedia,
"The concept of open source and free sharing of technological information has existed long before computers existed. There is open source pertaining to businesses and there is open source pertaining to computers, software, and technology."
"The open source model includes the concept of concurrent yet different agendas and differing approaches in production, in contrast with more centralized models of development such as those typically used in commercial software companies. A main principle and practice of open source software development is peer production by bartering and collaboration..."
I use and recommend Avira Antivir, the personal version is free. It's not true open source, as their business model involves selling muti-use licenses to businesses. It's their way of advertising, and it's fully functional, not vapor ware that stops working unless you pay. It's good.
http://www.free-av.com/en/download/1/avira_antivir_personal__free_antivirus.html (http://www.free-av.com/en/download/1/avira_antivir_personal__free_antivirus.html)
If your interested in turning your computer into a musical instrument a program who's development I was tangentially involved in, debugging and development there's a low cost program called FRACTAL TUNE SMITHY. There's a limited function free version, and an unlock key is also available as beggarware, barter, or artware trade.
http://robertinventor.com/software/tunesmithy/music.htm (http://robertinventor.com/software/tunesmithy/music.htm)