You can contribute to open source by using open source softwares. Some of the most useful computer software that you should try:
Firefox web browser: Browse the web with this awesome web browser. It's not only open source but also a product of Non Profit Organization (Mozilla Corporation).
Mozilla Thunderbird: Read and send email directly from your desktop.
RSSOwl Newsreader: Get latest news directly without searching or browsing websites.
VLC media player: Play your favorite music or videos.
GIMP: Edit or create photos, it offers many great features.
OpenOffice: Create and edit documents, presentations, spreadsheets and many more. It is compatible with file formats used by Microsoft Office.
Miro Video Converter: Convert videos to many different formats including Android, iPhone.
Miro: Awesome music and video player, allows you to search videos directly from it, browse websites, download torrents, listen to podcasts.
PeaZip: Create and extract archived files.
Notepad++ text editor: A very useful text editor that allows files to be opened in tabs, advanced search and replace, auto completion and you can customize it by downloading additional plugins.
Ubuntu Operating System: Linux distribution which is easy to use, configure and learn. If you wish to switch to Linux then it's simple with Ubuntu.
There are many free games that you can download and play. Some of them which I like are:
Supertuxkart: Solve challenges and compete with others.