Free as part of GOG's Black Friday sale.
To claim the freebie: Click "Get it here" on the game's page, then "Get it for free" in the free giveaway/MDK banner on the GOG homepage.
About MDK: As vast alien mining cities suck the power and minerals from every major metropolis on Earth simultaneously, time is of the essence as millions of civilians are expiring by the second.
Using your stealth parachute and the now infamous "Zooming" head-mounted sniper weapon, your task is to slip in undetected from the sky, seek out and remove the alien leader by any means.
Cheers. Used to love this game as a kid