Night of the Demon, classic.
Best lines:
Professor Mark O'Brien: Hobart, what is the order of the true believer?
Rand Hobart: Those of us who believe that evil is good and good, evil.
Professor Mark O'Brien: Yes, go on.
Rand Hobart: Who blaspheme and desecrate. In the joy of sin will mankind that is lost, find itself again
There's also the Quatermass films of the 50s and 60s. edit:already mentioned.
Got the free DVD of the original Godzilla film from the Guardian a few weeks back, haven't got round to watching it though.