1. Wikipedia
Changed the way we find information. Before Wikipedia, most online encyclopedias were either sorely lacking in information, or required you to have a paid subscription to access their content. Wikipedia changed all that by not only allowing anyone to view the content for free, but also by allowing individual users to review and update content, making it more complete and accurate overall. Wikipedia also brought crowdsourcing and user-generated content to the mainstream online, making both much more viable and valuable.