A variety of things: Books, movies, music, scientific journal publications, Slackware's "current" branch with all past packages since 2009 (only half a TB though), All Slackbuild sources, an almost-complete crawl of CentOS 6 packages, large language models and datasets (almost 8TB now), an old TankNet archive, a few wikipedia dumps about two years apart, chat logs, archived email, a lot of smaller archives of niche interests .. it's something of a mess.
Copies of websites that I find particularly interesting or enjoyable (Anders' Mage Page, ancientfiles.dumpshock.com). Source code that's gotten scarce or might in the future (e.g., yt-dlp, Ultrix V4.2 for VAXen) or is just interesting (Apple Lisa ROMs). Dumps of some Yahoo Groups I used to be on back in the day. News footage that tends to be difficult to find or might not get aired. A couple of text file CD-ROMs that I never managed to get hold of back in the day (e.g., Blackphiles Collection). Data dumps from some services I used to use (Twitter, Librarything). Datasets that I mess around with (Stop the Steal leak, covid-19 data). Stuff that just strikes me as odd or interesting (titor-special_torrent/).