Examples include:
Not all of these are found in the same type of cell. Most
prokaryotes lack organelles (by definition,
they lack a nucleus). Ignore
alex.tan, he's a medic :P
As for what an organelle is I suspect that being membrane bound is the best definition. I disagree with Gritchka; the ribosome is a cellular machine, and was never an independant entity. I guess it's not too important anyway.
See: Endosymbiotic Hypothesis