If I vote for a game on Steam Greenlight that I've bought on IGS and it gets accepted, Will I get a Steam Key?

This is really entirely up to the game's developer, but in nearly every case the answer is Yes, you will get a Steam key.

Our developer backend allows developers to upload Steam keys.  After they got accepted, the developer could request steam keys and upload them to IndieGameStand.  These keys would be available to anyone who purchased the game for more than $1. Bottom-line is that IndieGameStand cannot request Steam keys for developers, so the best thing to do would probably be to email the developer who has recently gotten on Steam and ask them to upload keys to our site.  If they get enough emails, they'll likely do it.  Indies are pretty cool like that.

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