Heinlein used the Anson penname whenever he had more than one story published in the same issue (usually “Astounding Science Fiction”). John Campbell got him to do this, because he didn’t want to look like he was padding the mag with nothing but Heinlein.
When John Campbell started out writing, his stories were in the pulp vein, a la EE “Doc” Smith. Later, when he began writing more thoughtful stories, he didn’t want his earlier pulp stories to color readers’ expectations. Don A. Stuart came from his 1st wife’s maiden name, Dona~ Stuart.
Asimov published his “Lucky Starr” juveniles under the pen-name “Paul French.” He later reclaimed them under his own name.
Harlan Ellison used the name “Cordwainer Bird” whenever he disowned a specific work. His most famous use was when Bird was listed as the creator of the TV show, “The Starlost.”
Some pen-names were used for collaborations. Examples include Eando Binder (Ed and Otto Binder), Lewis Padgett (Henry Kuttner and his wife, Catherine Moore), Robert Randall (Robert Silverberg and Randall Garrett), and Edson McCann (Lester del Rey and Fred Pohl)
Some authors used pen-names exclusively, like William Tenn (Philip Klass), Cordwainer Smith (Paul A.M. Linebarger) and James Tiptree, Jr. (Alice Sheldon). The reason for this is that they had already established non-SF careers (Linebarger was an Asian policy advisor in Washington, Sheldon a clinical psychologist), and didn’t want their writing to interfere with their chosen profession.
My favorite is Philip Jose Farmer who, when suffering from writer’s block, would pick a pen-name and start writing under that. He’s written as Kilgore Trout, Jonathan Swift Sommers III, Leo Queequeg Tincrowder, Rod Keen, Paul Chapin, and others.
I couldn’t find an on-line listing of pen-names, but I have a nagging memory that a book of this information exists somewhere. Wish I could remember more.