If you search for presentations, you should give a try to Slide Search Engine. According to information at the website the search engine has indexed above 8 million presentations and slide shows.
The service is describing itself by being an aggregator, which gathers publicly available presentations. Beside being searchable by its search function the presentations are assorted within numerous categories. A further most interesting feature are trending hashtags and trending tags, which enable to find presentations about popular topics: Slide Search Engine.