I know Monato; are there any other online magazines? And if so, where can I find an (exhaustive) list?
Not an exhaustive list, but there is a list of magazines you can subscribe to through UEA.
For french speaker, the association Espéranto-France put his revue on the net : http://esperanto-france.org/le-monde-de-l-esperanto
You'll found some article in esperanto and others in french.
Monato is a paper magazine. There are many online magazines, not so easy to find.
La Esperanto-centro: EVENTOJ. In this web page you can find several publications.
RET-INFO – Retpoŝta Esperanto-novaĵservo! (They will send it to you)
EVENTOJ - plena arkivo, all the magazines published between 1992 and 2002.
Juna Amiko – arkivo de 40 jaroj, all the magazines between 1974 and 2014. The paper version of this magazine is still being published. They will keep adding them ... 2 years late.
Internacia Scienca Asocio Esperantista, ISAE - kaj Scienca Revuo. Some articles from the paper science magazine.
Faka literaturo en Esperanto, S.T.E.B, huge collection of science books and articles, by subject.
La Karavelo, you can download 27 issues:
There is also a quite new magazine made by membres of the Telegram "Esperantujo" group, named Umujo : https://drive.google.com/drive/folders/0BwWhyDWcKt2VdWw5aTZ1b0Y4dGc