http://www.nasa.gov/columbia
NASA's Human Space Flight website has a condolence book to send your sympathy and thoughts of support to the friends and families of the lost crew members.
STS-107: Columbia and Crew Condolence Book
http://www.spaceflight.nasa.gov/feedback/condolence/index.html
Cards of sympathy etc. to the crew families and friends can be sent to:
Mr. Kent Rominger
Astronaut Office
Code CB
NASA Johnson Space Center
2101 NASA Road 1
Houston, TX 77058
USA.
Mr. Rominger served as the mission commander on the initial mission of astronaut Rick Husband, commander of the STS-107 mission. If you watched the memorial service at Johnson Space Center, he remembered the crew members and gave a wonderful insight into their personalities.