nvi[0] (New vi, BSD) by Keith Bostic uses bdb[1] (Berkeley Database) as it's backend, which is interesting. It may be because he (and Margo Seltzer) invented the bdb that it was on his mind, but interesting design decision nonetheless.

[0] https://en.wikipedia.org/wiki/Nvi

[1] https://en.wikipedia.org/wiki/Berkeley_DB