via Steamworks (no more complicated third-party tools). Steam Workshop support for easy modding. The Insect Invasion expansion included by default. How to Get It for Free (Legally)
While the game usually costs around , you can often get it for free or extremely cheap through these legitimate methods: Impossible Creatures Steam Edition Free Download
If you find a site offering a "Free Download" for the Steam Edition, proceed with extreme caution: via Steamworks (no more complicated third-party tools)