there are too many to name; MS Walk, Race for the Cure, Walk for Diabetes, Team in Training for Leukemia that Michael mentioned.
What I am getting very sensitive about is the percent that the charity actually receives out of the dollars that you donate, and here is a web site for you to cross-reference once you have settled on a worthy cause http://www.charitywatch.org/aboutaip.html.
Team In Training (http://www.teamintraining.org/) is a phenomenal cause worth donating money to! One of my clients trained for his first triathlon through them and has told me incredible things about the organization and its purpose.
Here’s a quote from Team In Training’s website to give you some insight into their cause: “In exchange for training and support, you help raise money towards cures for blood cancers like leukemia — the No. 1 disease killer of children — lymphoma and myeloma.”
You don’t have to necessarily sign up for one of their training programs to show your support either. From my understanding, anyone can make a donation.
Hope this is of interest to you!