The really obvious ones have been mentioned... How about
:
- Black Sheep
- Isolation
- Frogs
- Return of the Living Dead
- C.H.U.D.
- Mimic
- Larva
- Bats
- Severed: Forest of the Dead
- Cabin Fever
- Hills Have Eyes (to some extent)
- Jacob's Ladder (human atrocity/experiments against nature)
- Attack of the Giant Leeches
- Attack of the Killer Shrews
- Them!
- The Incredible Shrinking Man
As people mentioned, pretty much any Roger Corman or 50s-60s Giant Monster flick.