OAISTER is a database of Open Access content that includes scholarly articles, dissertations, theses, and archival materials. from hundreds of repositories. You can specify what type you are looking for.
Google Scholar searches scholarly content in myriad repositories and web sites. However, beware that it is the person who put up the content who determined whether it was scholarly. Much of it is, but there are also slide shows, undergraduate papers, and articles from low-quality and fraudulent journals mixed in.
The DOAJ is a directory and searchable database of thousands of Gold Road Open Access Journals. It has a large editorial board and rigorous selection criteria, which were developed after fraudulent Open Access journals became a notorious problem. After that, hundreds of journals have been ejected from the DOAJ for failing to meet those criteria.
