Well, since you love KeePassDX as me, I started to use KeePassDX as a “database of notes”. Under Root, you can create categories by adding 'Group’s. Inside each group, you can add an ‘Entry’ (a note). each Note, will have a sub Category (Title), subject of note (User name), url and the note itself (note). you can also add tags to your notes.
Give it a go, you’ll be surprised how well this works, finding/searching for whatever is there.

Great list, here are some more apps to consider to add to the list: InnerTune, Amaze, DiskUsage, DuckDuckGo, K-9 Mail, KDE Connect, Video Transcoder, Notally.