Dictionary.com defines a commonplace book as "a book in which records are made of things to be remembered". This is my commonplace book.
History
Writings and statements by others about history.
Security
Writings and statements by others about security.
Fiction
Favorite passages from works of fiction.
52 McGs
Notes from 52 McGs: New York Times obituaries.
Various
Notes, thoughts, and writings about virtually any subject, as long as it's interesting.