Factory
A factory is either a non-glider gun or a stationary Still life/oscillator maker.
The best-known factory is the queen bee, which manufactures beehives. Usually a factory is not stable all by itself, and requires an eater or other converter to clear its output between cycles.
Examples
See http://www.argentum.freeserve.co.uk/lex_f.htm#factory for information and examples.