Hola is the big one, but in practice, if we hypothesis that no one's running a VPN as a charity, free VPN products need to make money someway, and if you're not paying to use it, how else are they gonna make money?
So basically be suspicious of every single "free" or suspiciously cheap VPN. Go with known brands that come recommended by mulitple people, especially from people "in the know".
Though PirateSoftware (a person) has a good bit on why he doesn't advertise for VPNs on his channel.