Simple:
* The donor wished to remain anonymous.
* I received an anonymous letter in the mail.
More Complex:
* The anonymous tip led police to the suspect's hideout.
* The internet provides a platform for anonymous communication, which can be both beneficial and harmful.
* The article was published under an anonymous pseudonym.
Figurative:
* The city felt anonymous and overwhelming to the newcomer.
* The author's style was so anonymous that it was difficult to distinguish it from others in the genre.
Remember: "Anonymous" means without a name or identity.