Become a card-carrying Esperantist: Join the Universala Esperanto-Asocio!
(Disclaimer: I am not affiliated with the leadership of the UEA, nor am I acting on behalf of the UEA. This post is strictly my own opinion.)
The Universala Esperanto-Asocio is the worldwide organization of Esperanto speakers headquartered in Rotterdam. Founded in 1908, it exists to promote Esperanto as an international language, to act for the solution of language problems in international relations, and to promote better relationships among the peoples of the world.
The UEA is an organization, and like any organization, it has its share of flaws. However, the goal of the UEA is a very worthy one, the people who work in and for the UEA are good, conscientious people, and I think given the very reasonable cost (about $13 if you are in the USA) it's worth supporting.
You can also support your national Esperanto association (and most national organizations will allow you to purchase UEA memberships through them).
Benefits: Participating in something greater than yourself. A nice monthly magazine in Esperanto. Largest source of Esperanto books and materials.