When you need to use a physics engine, whether it is for an amazing game or just for learning, it helps to have choices. Here are the two different engines I have found to be useful. Both are free, and by the same company.
APE is the newer of the two versions. It is nicer looking too. It has a filled in colors and just looks cleaner. It also has some pretty nice features, like different types of wheels, springs, and collisions. The whole engine looks like a lot of time was put into it. It uses AS3.
Flade is not as new or good as APE and is programmed in AS2. It hasn’t been updated in years. That being said, it’s not horrible. It has the same basic features. It just isn’t as visually apealling.