Three apps I would really recommend are Evernote, Simplenote and Web of Trust.
I’ve used Evernote on Android, Fire OS, and on the desktop.
I’ve used Simplenote in the same places as well as in a mobile browser. I really wouldn’t recommend using it in a mobile browser though since it can be so slow. Maybe it’s just my phone.
Web of Trust comes as a browser add-on for Firefox, Safari, and Chrome on the desktop. You can also use it as a website. And the Android and iOS app are basically the website made into an app.
Debrezion Berhe Gebre
Update 5-15-18: There is no Web of Trust mobile app for iOS. You can still just use it in a browser in iOS though.