Steel Panthers IP is handled (this is where I screw up and give wrong info) by Matrix Games. If Matrix doesn't own it then at least they're the ones handling distribution. A few years back a couple SP variants were available for free on that site, but they sure were a challenge to find on there. These free versions did not include any Mega Campaigns but came loaded with a ton of scenarios.
Ahh, I see on the site that it's been discontinued. Err, the free basic version, that is. You can still get the big Generals Edition there.
http://www.matrixgames.com/products/297/downloads/ Given the $70 price tag, I don't suppose we'll see that version any time soon. Who knows, maybe the previously free original will make it to GOG someday.
There was a modern warfare version that I can't find anymore, also for free several years back. Maybe it got rolled into the Generals Edition.