Steam

This. So much this. Monopolies are often evidence of an unhealthy/stagnating market, but they're more symptom than cause. Trusts/cartels, price-fixing, and anticompetitive behavior are the actual abuses of market power, and are much more problematic.

I'm not going to claim that Valve is perfect (they're not, e.g. see issues regarding DRM) and I'd love to have more choices about where I buy my games, but I can't think of any instance of them abusing their position in the market to prevent new entrants or claim an unfair advantage. From what I've seen, they appear to be a very fair and honorable competitor in the space. However, if anyone is aware any examples to the contrary, I'd love to hear about it and update my opinions.