Formal:
* Aussi connu sous le nom de... (literally: Also known under the name of...)
* Connu également sous le nom de... (literally: Known also under the name of...)
* Surnommé... (literally: Nicknamed...) - This is more formal and can be used for a person or a place
Informal:
* Appelée aussi... (literally: Called also...)
* Surnommé... (literally: Nicknamed...) - Can be used in a more informal way for a person or a place
* Ou encore... (literally: Or again...) - This is a more casual way to say "also known as" and is often used in lists
Example sentences:
* Le poète Charles Baudelaire est aussi connu sous le nom de "le maître du spleen". (The poet Charles Baudelaire is also known as "the master of spleen.")
* La ville de New York est connue également sous le nom de "la Grosse Pomme". (The city of New York is also known as "the Big Apple".)
* Le chien a été surnommé "Max". (The dog was nicknamed "Max.")
* Ce plat est appelé aussi "poulet à la crème". (This dish is also called "chicken with cream.")
* La ville a une population de 100 000 habitants, ou encore 100 000 âmes. (The city has a population of 100,000 inhabitants, or again 100,000 souls.)
The best way to choose the appropriate phrase will depend on the tone and context of your writing or conversation.