Erlang is an amazing language for learning how to effectively use OOP principles. I feel like anyone who works primarily with class-based languages owes it to themselves to write a few server-like modules in Erlang even if they never touch the language again.
Perl is also another interesting language for learning the low-level details of OOP, since its default set of OOP features (as of Perl 5; this all changes in Perl 6) involves manually "blessing" some data structure into an object. Objects are thus just "blessed" data structures (and not just hash tables, either, though those are the most common), and methods are just subroutines that accept their object as their first argument.
Between Perl and Erlang, I think I've ended up learning way more about real object orientation than I did from my dabbling in C++ and Java.
I have been thinking for a while that oop in the message passing sense is basically coroutines. I guess that is encoded in Erlang? Probably should learn it at some point.
