The Twin Towers, destroyed in the 9/11 attacks, serve as a remembrance of that day and the lives lost. One design option would be the towers with an American flag behind them, to show the strength the country had after the attacks. A creative way to incorporate the towers into the date is to use the towers as the 11 in 9-11-01. "Gone but not forgotten" would be appropriate text for this kind of tattoo.
A stand-alone American flag is another meaningful way to commemorate 9/11. An American flag shows patriotism and love for the country. If you want your tattoo to be more specific to 9/11, you can include the date, the towers or both.
The bald eagle is a symbol of American strength and unity and makes for a beautiful tattoo. You can get a lifelike rendition of the animal or the Great Seal of the United States, which features a bald eagle holding 13 arrows and an olive branch. This symbolizes that the U.S. desires peace, but is ready for war if necessary. Either one of these would work as a 9/11 tattoo, especially if incorporated with the date of the attacks.
Many people like to remember the attacks by honoring the men and women who put their own lives at stake to save others on that day. Tattoos that read NYPD and FDNY are very popular and can be beautiful when combined with American flags and bald eagles. Symbols such as firefighter helmets and police hats are also a meaningful way to represent those heroes.
Crosses and angels can be used to represent hope and remember lost loved ones. Combined with some of the other ideas, these can be powerful displays. If you lost someone close to you on 9/11, you can personalize your tattoo with the name or birthday.