Currently looking at the concurrent gamers stats, when KSP came out of beta it spiked to over 19000 concurrent players. I personally can't get enough of the game.
Speaking of which, if someone takes time and pairs the data from Steamspy with SteamCharts, I'm pretty sure you'll be able to come up with some incredible stats, maybe showing how many of the games actually bought during the Steam sale is being played, and how much does the average Steam sale boost player activity. I would love to see stats like that.
Also, on KSP, check out the
Kerbal Space Program subreddit. It's probably the friendliest subreddit I've ever seen, and they are super inclusive of new players, in fact, they invite newbies to post questions and screenshots. For videos, just Google Scott Manley