Wikipedia is probably the most well known and used example of open source ideology. The website is an online encyclopaedia that can be edited by anyone. As a result it now has millions of pages in nearly 300 languages, with around 100,000 regular contributors. As it is open for anyone to edit, it does leave the website vulnerable to errors in the accuracy of the content, and also vandalism. Despite this it is an excellent resource for finding starting points for research, as there are often citations and references to reliable sources that the information has been retrieved from.
Wikipedia even has pages in rarer languages such as Kalaallisut (Greenlandic), showing just how much it has grown over the years. Although it doesn't necessarily include many pages in uncommon languages, they are increasing all the time.
YouTube is a great example of how people can share online. The video sharing website was founded in 2005, and since then has become one of the most visited websites in the world. Users can upload their own videos about anything and everything (there are limits of course), creating a huge network, and even its own community. It can be used for educational, social, entertainment and artistic purposes, enabling an online culture of sharing and collaboration that wasn't previously possible.
Open source can be applied to games and simulators as well. One example of how it can be used is openBVE, which is an open source train simulator. Developers build routes and trains, which users can then use to "drive" a train.
Even food and drink can be the subject of open sourcing. OpenCola is an open source drink where the recipe can be modified further. The only rule is that the recipe (which is freely available), once it has been modified, then has to be re-licensed under the GNU General Public License.
Girl Talk is a musician who specialises in music sampling and mashups. He makes new music from existing songs and releases them as LPs, under a Creative Commons license allowing people to then use it to make their own work. He also lets people choose how much they want to pay for the albums, something which other music artists have also started doing more recently.