The main one I'm thinking of is monotone, which is a distributed version control system that started to gradually lose activity after git was released and took

The main one I'm thinking of is monotone, which is a distributed version control system that started to gradually lose activity after git was released and took over. The primary conceptual difference I think, is that git is federated-like-email while monotone is distributed-like-usenet. It's a far prettier model, but it makes getting consensus on who has commit privileges (or in equivalent modern terminology, what blockchain appends are valid) somewhat of a PITA (especially when the list of approved committers can change over time). We never did get that working completely, but the work done since on bitcoin probably has some useful insights. A couple other things to keep in mind, are that innovation is not quite the same as invention; and that all inventions build on previous knowledge.