This article is from the Object-Oriented Technology FAQ, by Bob Hathaway firstname.lastname@example.org with numerous contributions by others.
Simula was the first object-oriented language providing objects, classes,
inheritance, and dynamic typing in 1967 (in addition to its Algol-60 subset).
It was intended as a conveyance of object-oriented design. Simula 1 was a
simulation language, and the later general-purpose language Simula 67 is now
referred to as simply Simula. Smalltalk was the next major contributor
including classes, inheritance, a high-powered graphical environment and a
powerful dynamic typing mechanism (although these existed to some extent in
Simula). Self is somewhat of a Smalltalk-based next generation language, as is
BETA a followup to Simula (by its original designers).
[Meyer 88] contains a brief summary and history of Simula and Smalltalk, among
other OO languages.