Here is one of the more creative ways that you can commemorate the victims of 9/11 on Sunday. The Aero Pines Park community in Second Life are sponsoring a "Stairclimb Firefighter Tribute 9/11 Remembrance" on September 11. You can outfit your avatar in firefighter gear, grab a nozzle, and ascend 110 stories of a simulated World Trade Center Tower to honor the fallen firefighters and EMTs who lost their lives on that day.
To participate, teleport to this location, click on the free firefighter outfit, and get climbing.
Might I also recommend that you head to Broadcastr to listen to anecdotes from first responders who survived, and others recounting their perspectives on what happened on that fateful day.
UPDATED 9/12/11: Here's an interview with project coordinator Cindy Bolero after the jump...