Can anyone tell me which Star Trek movie had the scene where a "bomb" of sorts is dropped on a planet - and instead of destroying - it creates life in atomic-bomb like fashion?
Star Trek II: The Wrath of Khan. Also Star Trek III: The Search for Spock.
Khan seeks revenge on Kirk for stranding him on Ceti Alpha V (events depicted in the TOS episode "The Space Seed"), ends up killing everyone on a research station and stealing the Genesis bomb, which Khan uses as a last ditch attempt to kill Kirk after the Enterprise manages to defeat Khan's ship (the USS Reliant, which Khan stole from a research team that landed on Ceti Alpha V). Spock sacrifices himself to repair the reactor so the Enterprise can escape the blast radius.
The Genesis bomb creates a new planet that is explored in Star Trek III.