The Internet's largest online support group for people with Lymphoma, their friends, and family, can by found at
http://forums.lymphoma.com.
With over 500,000 messages to date (over 150,000 in the original 1990s system and nearly 400,000 in the current system), this system has been an invaluable part of many people's lives since 1996.
While traditionally focusing on Lymphoma, a March 2009 expansion adds specific support for Leukemia and Myeloma.
Use of the message forums is entirely free. Support to keep them running is provided by WebMagic.
Wikipedia links:
Leukemia:
Lymphoma:
Multiple myeloma