opsdo has two goals - to allow people to search many information sources which mainstream search engines do not cover well and to allow people to search groups of such sources -- e.g. RottenTomatoes, IMDb and Amazon.
OpenSearch is a set of simple formats for the sharing of search results. Any website that has a search feature can make their results available in OpenSearch™ format. Other tools can then read those search results. For example, here is an aggregator tha