Biblical commentaries may come in various formats. Many commentaries that were published before 1924 may be freely available on various websites or through Google Books. For commentaries in the Public Domain (i.e., published before 1924), see the section below.
More recent commentaries are generally not available freely online because they are still under copyright. To find these in through the library these guides: Print Commentaries and Online Commentaries.
There are a number of websites and projects that attempt to collect and provide access to biblical, theological, and religious resources that are freely available online. The majority of the resources contained in these collections are in the public domain, meaning that most of them were published prior to 1924.
Many Bible study websites include several commentaries from the public domain to supplement the biblical texts. Here are the most robust and useful Bible study websites:
These collections list and link to commentaries and commentary sets in the public domain: