I use Apple's Notes to keep structured notes in. I use Notes in preference to other personal management software because it is free, and the replication between the Mac and iPad and iPhone works pretty well, and it has enough features for me.

One downside of Notes is not being able to backup the data in it. If the data in Notes is local to the Mac then one can find a sqlite3 database buried inside ~/Library/Containers/com.apple.Notes. But that doesn't contain the data held in iCloud.