tfishell: The Suffering - published by Midway, made freeware by the U.S. Air Force ... ?
http://en.wikipedia.org/wiki/The_Suffering_%28video_game%29 Looks like a great survival-horror game, not to mention it originally sold 1.5 million copies, but it seems like quite a bit of work to try to get the re-release rights. (Related: I believe Area 51, also a Midway game, was also released by some U.S. military branch as freeware.)
SCPM: Another Midway game was released as freeware, Psi-Ops: The Mindgate Conspiracy:
http://www.ausgamers.com/files/download/36534/psiops-the-mindgate-conspiracy-free-game http://www.atomicgamer.com/games/5593/psi-ops-the-mindgate-conspiracy/files The functionality is supposedly restricted to users from the U.S. and Europe, so download at your own discretion. I don't know what the deal with Midway is, though.
I'd personally love to have GOG put up Cave Story, Major Stryker, Bio Menace, and
DBQuest. ;)
I can't say if these were ever released for free, but they were made available as adware by Midway, with DRM by Double Fusion. The company went tits-up a couple years back, so there's no real ads to be seen in the games, but the DRM remains (thankfully, the servers seem to remain, too). I've found workarounds for Area 51 and The Suffering, but they're probably not legal. Regardless, if the rights to these do get resolved and they get re-published, I doubt they'll be free.