looking around for a requirements management tool, preferably a general
product development tool rather than very SW development orientated, and
preferably open-source.

Those that I have found seem either at best dormant and often dead and
none appear to have much maturity. I was hoping to find something
comparable with Bugzilla in terms user base and maturity

What do the big projects use? Any recommendations (including commercial
offerings)?

Thanks

Stair