I have redeemed codes from Humble Bundle and a couple of kickstarters on Steam. Then there's the few freebies and a couple of gifts. I wouldn't buy retail that says Steam in the back, that's pointless. I mostly use Steam for convenience, achievements and to keep in touch with a friend who lives in TF2.
While I've only bought bundles with DRM-free downloads, some of them have come with extra Steam-only keys that I have also redeemed. On Steam itself I haven't spent any money, I don't even have a wallet.
I randomly got some valuable gear on Team Fortress 2 that I traded for a premium upgrade. I suppose I could in similar manner consider earning some
play money and spend it on some games, but I'm bored with TF2 already, so probably won't. Had they recently made life hard for old accounts too that haven't spent a $5, I guess I could have gotten some game listed on
this thread to get around it.