Norway’s sovereign wealth funds might be what you’re thinking of? It’s pretty awesome. Norway’s state owns a fund equal to nearly triple their GDP and amounts to over half the country’s total wealth. Think about that: half the nation’s wealth taken out of the hands of private capitalists! If you exclude real estate, then Norway’s government owns 3/4 of the nation’s wealth.

As mentioned earlier in the comments, the PRC is trying this as well. The Chinese government owns about a third of China’s wealth (as of 2018).

The US state of Alaska manages their oil with a sovereign wealth fund, and pays a dividend to its residents every year. Because of this, Alaska almost always ranks #1 or #2 in the US for lowest wealth inequality. And that’s with Republicans typically in charge!

Matt Bruenig has a white paper on sovereign wealth funds. They’re basically a market socialist’s wet dream: https://www.peoplespolicyproject.org/projects/social-wealth-fund/

Sweden tried it as well but a right-wing government fucked it up in the 90s.

Sweden: https://en.m.wikipedia.org/wiki/Rehn–Meidner_model

Also here’s a good article on it: https://www.jacobinmag.com/2017/08/sweden-social-democracy-meidner-plan-capital

EDIT: Sort of related, Norway does have sovereign wealth funds that own large portions of the national economy in a similar way, though it still falls far short of anything resembling full socialism: https://en.m.wikipedia.org/wiki/Government_Pension_Fund_of_Norway

The PRC has a similar fund, which is relatively new: https://en.m.wikipedia.org/wiki/China_Investment_Corporation