The Guggenheim Library's "HawkFind" search tool labels OA articles as such.
The Directory of Open Access Journals provides browseable lists of journals in many subjects.
OAIster is a union catalog of millions of records representing open access resources. It was built by harvesting from open access collections worldwide. Today, OAIster includes more than 25 million records representing digital resources from more than 1,100 contributors. The records for these items are included in OCLC's Worldcat, but if you want to search just for open access content, you can do that through a separate OISter interface.
FlourishOA is a data-driven web app and API enabling users to discover relevant and reputable Open Access (OA) publications in order to maximize publishing impact. By aggregating price information and impact data, it empowers researchers to identify credible OA journals that best fit their publication needs.
BASE (Bielefeld Academic Search Engine) is an online search tool for open access resources and documents. Hosted at the Bielefeld University Library in Germany, BASE harvests its data from open access scholarly repositories.
Although it is not possible to limit search results to text materials licensed for re-use, Google and Google Scholar both index a large number of repositories and return open access results.
