On October 23, 1983, Islamic terrorists drove a truck bomb into a building in Lebanon housing U.S. Marines on a peacekeeping mission. The attack killed 307 people, including 220 U.S. Marines, 18 U.S. sailors, and 3 U.S. soldiers. It remains the worst act of terrorism against American citizens on foreign soil.
On February 26, 1993, Islamic Terrorists bombed the basement in the WTC in hoping to knock down one tower over the other and thus killing thousands of people. That really failed because the bomb wasn't as powerful to knock down the dent steel within the parking garage and only ended up killing six people.
On April 19, 1995, right-wing terrorist Timothy McVeigh attacked a federal building in Oklahoma City, bringing down the whole building and killing 168 people, including 19 children. It remains the worst act of domestic terrorism in American history.
Other than that, no terrorist attack killed thousands of people on a single day like 9/11 did.