Coming up on the 10th anniversary of the 9/11 attacks, I found a good patch in the Embroidered Patches section at The Cheap Place. It's one of their fireman patches that have been given the name FDNY MIA Never Forgotten, and it commemorates the fallen firefighters who died that day in lower Manhattan. Although there were police officers and other rescue workers who also died, this one simply honors the fireman.
It's a white-on-black, square patch which is reminiscent of many of the popular POW-MIA patches former military personnel wear. It features a silhouette of a firefighter against the backdrop of the twin towers and the 9/11 date. It effectively conveys the message without being overbearing or offensive, and makes a good commemoration of the day if you're a firefighter or family member of one. Perhaps if you have an FDNY t-shirt you plan to wear on the 10th anniversary, this patch would be appropriate to attach to it.
It has an iron-backing which allows you to "glue" it to cotton, denim, or most other fabrics. Just don't try to iron it to leather. For a leather jackets and vests, you'll have to employ good, old-fashioned, needle and thread to attach it. If you would prefer not to affix it to your clothing, you could put it on a backpack, a purse, a baseball hat, your tackle box, or anyplace else you have some open space. Be creative - I'm sure you could find a place for it.
