Informal:
* MDR: This stands for "Mort de rire" (literally "Death of laughter"). It's very popular and used similarly to "lol."
* mdr: The lowercase version is also common.
* LOL: The English "LOL" is also used by French speakers, especially younger generations.
* pdr: This stands for "Pété de rire" (literally "Farted from laughing"). It's a more informal and slightly vulgar option.
* XD: The emoticon XD is used, often with the meaning of laughing so hard you're crying.
More formal:
* J'ai ri: ("I laughed")
* C'est drôle: ("It's funny")
The best option to use depends on the context and the person you are speaking to.