There are lots of ways to help online. One easy way is to join the "Answers for the Cure." http://www.ask.com/cpp?qsrc=4117 For every answer you contribute, Ask will donate 10 cents to Susan G. Komen for the Cure. There are similar ways to donate online for free at places like SocialVibe and FreeKibble.
Years ago, I used to volunteer on a website that had message boards and chat rooms. I hosted several message boards and a chat each week helping people with issues that I felt were important. Look around for volunteer leader or host positions on the websites you frequent.
You can also participate in online activism. Depending on what you are passionate about, there are plenty of causes to join which allow you to email your political representatives or businesses in regard to various issues.
Start a website or a blog and use your skills to help people.