Open Source technologies or computer software grants users the rights to use, study, change and distribute the source code. Source code can be used to redistribute by anyone for any purpose. Open Source Programs / Software are very popular now a days because its cost effective solutions.
So many examples are around us. Operating Systems like Linux, UNIX, PHP, apache etc. PHP help us to develop software, websites and open source other frameworks. WordPress is a very common example.
Here is the list of most common available open source programs and software.
eCommerce Platform like Magento, OsCommerce, WooCommerce.
Android by Google
WordPress is the most popular CMS (content management systems). You can manage blog and also developed website, ecommerce or even application in WordPress Open Source CMS.
Odoo ERP it has two edition one is community which is open source and the other one is Enterprise edition.