Having used it a few minutes, DEVONsphere Express is super cool. It hangs out in your menu bar until you drop down the window, then it finds things related to whatever your frontmost active window (Brave, Finder, etc.) is viewing.
It has a few idiosyncrasies, like working better with Mail than MailMate, sometimes saying File not Indexed, etc. This may well be due to a gap in my knowledge, like needing to index the MailMate email folder.
Overall it looks to be a good way to get DEVONthink’s See Also functionality outside of DEVONthink.
The example shows an email from a mailing list I subscribe to. It found one of the authors in past emails. I clicked the little Left arrow, and it found the author’s paper in my Bookends and Zettelkasten databases.