https://galois.com is employee-owned, and AFAIK they use interesting technologies like Haskell, theorem provers, etc.

Would be interesting to structure a software startup as a coop.

I think the pushback would be the founders and early employees willing to share equally with later employees/members.

Igalia is that, in Spain:

http://www.igalia.com/

There was one in my hometown, in a different industry. One thing I heard about it was that decision making was extremely painful.

It wouldn't surprise me if there weren't similarities to volunteer organizations, decision making in which--as you say--can be very painful.

Valve is basically that, no?

No. Valve is privately owned, with GabeN having a big share (AFAIK, since it's private, we wouldn't easily know who/what owns each % of Valve)

You might hear about Valve in these kind of discussion not because of its ownership model but because of its "flat hierarchy", the philosophy of "no boss". But obviously, humans always organise in hierarchies, companies like Valve simply have no formal ones, you just have to navigate well social/politically to understand who you need to please.

Also, a single person can't usually deliver a big project in a reasonable timescale, so how do you get a team?

There's always a lot of more politics (arguing around, convincing others) involved in flat-hierarchies or co-ops, but they are much less likely to get a power hungry vulture dictating things.