"J.S. Mill, Market Socialist"

That's Chris Dillow's description at Stumbling and Mumbling. Consider this quote from Book IV, Ch. 7 of Principles of Political Economy:

The relation of masters and work-people will be gradually superseded by partnership, in one of two forms: in some cases, association of the labourers with the capitalist; in others, and perhaps finally in all, association of labourers among themselves....The form of association...which if mankind continue to improve, must be expected in the end to predominate, is not that which can exist between a capitalist as chief, and workpeople without a voice in the management, but the association of the labourers themselves on terms of equality, collectively owning the capital with which they carry on their operations, and working under managers elected and removable by themselves.

Dillow comments:

Mill's vision here is not that of state-owned enterprises. Government management, he said, is "proverbially jobbing, careless, and ineffective.". Instead, it's a vision of worker co-ops. And what's more, of co-ops that compete against each other....

Interestingly, Mill's market socialist aspect was the basis of the official ideology of a 21st century superpower in Ken MacLeod's novel, Cosmonaut Keep. The Communist Party of the European Union was inspired less by Marx than by Mill's vision of the "steady state economy":

"But the Party!" she said. "How can you stand that? I mean, nobody believes in Communism any more, not even the commies."

"Oh yes they do," I said. "They just don't call it that. They call it 'the sustainable society.' What economists used to call the stationary state. And they think they're getting us there, and that everybody will get there in the end, even the Americans."

"Never!" Camilla said. "Maybe the East Coast liberals might go for that, but not the rest of us."

I sighed. "It's got nothing to do with what anybody believes in. The falling rate of profit will get you in the end. You can evade it for a while by exporting capital, and follow that falling rate like a star sinking on the horizon..., but all that gets you is a fully capitalist--and fully capitalized--world, with low profit rates everywhere, and then there's nowhere else to go but the steady state, an economy just quietly ticking over rather than expanding. In the steady state it's easy for workers to end up employing capital--socialism, as near as makes no difference.

She shot me a suspicious look.

"This is Marx, right?"

"Wrong," I said. "It's John Stuart Mill."