Project Nomads is a computer game by Radon Labs released in 2002. It is set on the fictional planet Aeres, a fantastic world of gigantic floating islands, which are the remnants of the planet from before it exploded. After Aeres broke apart, the few surviving inhabitants settled on the drifting fragments of the planet. You are a nomad, a restless soul forever in pursuit of adventure. You wander across the planet on your island, discovering mysterious artifacts and exploring the ruins of the long dead race of Master Builders. The only place to fall to is the Sea of Flames, the very bottom level of this new world. Source: wikipedia
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I'm not sure if you'll be able to find a game that plays and looks like Project Nomads.
This game combines a high number of mechanics from different genres into one, very cool steampunk action adventure RPG with elements of RTS.
Moreover, you can choose a character you play that affects several in-game mechanics like types of weapons, aircraft and difficulty.
For new players, maybe there are several bad sides such as small campaign and square graphics. But it should be mentioned that graphics can be enhanced by several features as widescreen and GUI fix, ambient fix or even 3D mod. And there is a huge mod called Project Nomads Resurrection that adds changes in main campaign, making it harder and opening more content. (instead of )
Therefore, I recommend everyone play it through at least one time in original campaign to feel what it's like to be in post-apocalypse world of islands.