Tweensy is an extremely efficient Actionscript 3 property tweener. Contained within Tweensy is an expansion package called Tweensy FX which allows for motion effects to be applied onto DisplayObjects.

Tweensy is provided as an open source project but your small donation helps us to continue its development.

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Tweensy Visit this group

Tweensy is currently in a public beta stage of development.The latest source is available in the SVN, though older versions can be found in the downloads area which are updated from time to time.

If you are new to Tweensy read the wiki page here to get started.

Documentation can be found on each of the Tweensy products here
Tweensy Zero
Tweensy Original
Tweensy FX

To see how Tweensy compares to it's next closest tween engine in regards to performance not only in fps but also its stable memory usage.
These tests include 500 - 5,000 DisplayObjects (warning this may max out your CPU) animating at any one time. The result is Tweensy can be up to 10fps faster or 1.5 times the frame rate of TweenMax - wowzas!
Though Tweensy is overall very efficient these frame rate improvements are best achieved by using the BitmapLayer class to draw to the stage available in TweensyFX.
Constant tween test
Bulk tween test

Tweensy FX Examples (contained in the SVN - examples/fx)
Fire
Alien Rain
Bulge
Bump Map
Electro Bolt
Fire Smoke
Magic
Orbiting Magic
Pink Blobs
Pink Blobs 2
Pixelate
Smoke
Zany Bolts
Line Effects