While Ravel Law does operate as a subscription-based platform, some of its features, including its case law database, are available for viewing for free online (downloading the case is disabled unless you subscribe, however). It also offers additional features you would not get from the other free sources, such as the ability to visually map out references of a certain case:
Hovering over the different case circles will allow you to visualize which cases refer to which others. You can also narrow by date range or by court, and clicking through to the case on the right will open up the full text for you to view.
While some of its free features are limited, Ravel Law still seems to be a great source for those looking for free access to American case law, and its search visualization features are definitely fun to play around with!