Well you didn't really add any context here.<p>Do you want to see open source software or just software in general?<p>If its the latter then I'd put a vote in for the Island ECN. <a href="https://en.wikipedia.org/wiki/Island_ECN" rel="nofollow">https://en.wikipedia.org/wiki/Island_ECN</a><p>The Nasdaq bought them in 2002 and they became the basis for their technology stack. It's where we got lovable names for apis like ITCH and OUCH. They were doing things like distributed computing back before it was cool.