OWASP (The Open Web Application Security Project) is a worldwide not-for-profit organisation focused on improving the security of software. With over 200 local Chapters worldwide and 45,000+ volunteers OWASP's open community is dedicated to enabling organisations and individuals to develop and maintain applications that can be trusted. OWASP's meet-ups, tools, standards, guidelines, documents and forums are free and open to anyone interested in improving application security.
We usually run 8-10 events per year: meet-ups ("Chapter Meetings"), Capture The Flag (CTF) tournaments, Hackathons, workshops and joint themed events with goups like BCS, IISP & ISC2.
OWASP is essentially free to access but if you're feeling altruistic, there is a paid membership, which is a donation to the OWASP Foundation and gives you discounts on many cyber-security conferences around the world, voting rights, @owasp.org email address and many other benefits. Join OWASP Global Foundation as a paid member here: https://www.owasp.org/index.php/Membership