All kinds of things here. Many of the transfers are ineptly done, and there are occasionally things here that aren't 78-era at all. But there are thousands of files to download, in mp3 and other formats. Do check the subcollections in the left hand window. The Archive had committed to eventually digitizing the entire corpus of 78RPM recordings, so worth looking.
American blues and old-time music, Cajun, gospel, jazz, Mexican, calypso from Trinidad, Irish and more. The owner died in May 2015; the family pledges to keep these pages up, but they may be on borrowed time.
Almost anything can be found here, but not everything posted is strictly in the US. You may find a YouTube to mp3 converter an aid in working with the audio, such as MediaHuman's YouTube to mp3 converter. (https://www.mediahuman.com/youtube-to-mp3-converter/)
From the Library of Congress and with the permission of Sony, here are a ton of streams of acoustically-recorded Victor records (with other Sony-owned labels to come) Note the disclaimer about offensive language (primarily "the n-word"). Includes a very nice search engine.
Naxos offers the world's largest online classical music library, with streaming access to more than 120,000 CDs of both standard and rare repertoire in all genres. Users can listen to individual tracks, entire CDs, or create custom playlists.