Google Scholar is a free full-text search engine like Google, but focuses on scholarly content like journals, books, and dissertations. You can also expand the search to include patents. The resources you find in Google Scholar may be paywalled, but the searching itself is free.
There are often multiple versions of the article linked from the results, increasing the chances that you will find an available copy. These other copies may be preprints, or copies deposited in institutional or government repositories.
Google Scholar searching will cover many of the resources on this guide, including the publisher websites and preprint servers. While Google Scholar does offer some limited advanced searching options (see below), you will find better options searching directly searching the journals on that publisher's site. The tradeoff, however, is that you are only searching one publisher's journals.