Currently a member of the ACM, because a $75 membership gave me a $150 discount on a registration for a conference. I've been a member before for the same reason, but always let it lapse if I'm not going to any conferences. And given how absolutely ludicrous their conference prices are now (well over $1,000, plus travel and accommodation).

I'm also a member of the CHERI Alliance. It doesn't cost anything for individuals (though I'm now a corporate member, which does cost money) and is useful for the the various working groups and so on. There are a few other similar industry groups focused on specific technologies.

I'm a member of the BSI's delegation to ISO JTC1/SC22/WG21. Which is the official name for the C++ standards committee. Joining that is useful if you want to get anything into C++, though mostly I've spent my time trying to stop things going into C++.

I'm a member of RISC-V International (both as an individual and as the voting member for my company, which confuses their systems a lot). They're part of the Linux Foundation, which is probably maintainer of the worst infrastructure that I've ever seen. I would strongly recommend that any foundation looking for an umbrella organisation avoid them. RVI is an organisation that is still trying to evolve its processes and is reinventing a lot of the mistakes that other standards groups have made. It may eventually become a useful thing.