Three main reasons why I am a TEDster (a member of the TED community) are:
Even though the content of the conference eventually makes its way online the live experience is much better. Experiencing them together with others really enhances the experience. There is a big feeling of excitement and community during the conference and you can immediately have discussions about what is presented including with the speakers themselves.
TEDsters are a great bunch of people to connect with. During the conference you can dive into a deep conversation with any other attendee or speaker without hesitation. "In real life" I find that I first have to gauge if the other person is interested, willing and able to dive into a deep discussion.
Great connections can be made. Socially, but sometimes for business as well of course. I've connected several TEDsters to other people in my network for various mutual benefits.
I've been looking for bigger causes to contribute to than local volunteering and ad-hoc contributions to organizations like the Red Cross. It can be difficult to find a charitable organization that you can identify with and that you can trust. TED, owned by the Sapling foundation, is such an organization for me.
The TED Prize, the TED Fellows program and the TED Talks online are examples of how TED supports good causes. A portion of the membership fees get directed to these causes and TEDsters are encouraged to help out in other ways if they can.
This year (2009) I've started to translate TED talks, pledged to mentor (a) TED Fellow(s), pledged to spread the message about SETI and helped a bit with the Charter for Compassion by doing a translation. Especially for the Charter for Compassion I hope to find more opportunities to contribute (as an atheist) and I'm always on the lookout for more.
(You don't have to be a conference attendee to help if you would like.)