General usage:
* supplémentaire (most common)
* additionnel (less common)
* complémentaire (if it refers to something that completes something else)
Examples:
* Additional information: Informations supplémentaires
* An additional cost: Un coût supplémentaire
* An additional room: Une chambre supplémentaire
You can also use the preposition "en plus de" (in addition to) to express "additional" in French.
Example:
* In addition to the main course, we can offer you a dessert. En plus du plat principal, nous pouvons vous offrir un dessert.