JamesGecko: I'm surprised nobody has mentioned Unity. It's free (ish), it supports C#, and although it's clearly designed for 3D games, you can definitely make axis-constrained 3D games with it relatively easily. 2D sprite-based stuff, even.

JamesGecko: I'm surprised nobody has mentioned Unity. It's free (ish), it supports C#, and although it's clearly designed for 3D games, you can definitely make axis-constrained 3D games with it relatively easily. 2D sprite-based stuff, even.

orcishgamer: Unity is not free at all if you want to deploy to a mobile platform. You can use ShiVa for free for mobile game development, not sure how sidescrolling development works on it though.

captfitz: the first tutorial is about how to make a platformer, in fact
unity is a lovely thing