I've been sitting on this project for at least 2 3 years now, I never really got around to publishing it, but here you guys go:
https://github.com/athairus/airshot-trainer
What is it? It's a Sourcemod plugin that draws a handy target in the exact place you should shoot to score an airshot. I think a video does it more justice than a text description:
http://youtu.be/x6XfEhjpnv8
It's far from done, though. Besides the todo list at the bottom of the plugin, a big thing that needs to be done is to support showing the correct target to more than one person. I'm imagining MGE with all players having this to assist their aim. Problem is in Sourcemod, you can send sprite entities to each player whose positions are not interpolated, or create entities that have the same position for all players, making them useless for all but one.
A more elegant solution to this would be to ditch Sourcemod altogether and have it done client-side. I highly doubt Valve would sign such a plugin, so it'll have to be launched in -insecure mode to work at all. There's no reason to be concerned about a VAC ban if such a plugin were made.
Anyway, I figured instead of sitting on this plugin any longer I should just let the community be aware such a thing is possible and see if someone can take up the mantle.
Programmers, adopt this puppy plugin!
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EDIT: I seem to be getting a lot of questions about whether this is a cheat or not. Of course it is! It's not an aimbot, though. It only tells you where to aim. But you won't be able to load this in your local client, it's meant to be run on the server. So, the only way to even get this working is to play on a server that has this plugin installed.