MegaGames has reposted my game executable on their website without permission.

What do I do?

itch.io link: https://antzgames.itch.io/raid3d

MegaGames link: https://megagames.com/freeware/raid-bungeling-bay-3d-free-full-game

Try to write to their stuff: megagames@megagames.com

and say that you have no pleasure to have your game in their site. You can also invoke the DMCA Act. In your game is always recommended to write a link to the itch.io page.

• By posting their link here, and in full, you're driving more people to their site., which will make their site more popular and worsen the problem.
• You can issue them with a take down notice - they'll probably ignore you.
• You can contact their ISP and report them for stealing games. Otherwise report their site whenever you see, for example, if it comes up on a search engine listing - just report it. 
• You can shrug it off and carry on. It sucks that other people are making money from our games when we've not earned anything from the games ourselves - but that's the reality that is the internet right now.