Check out the demo!

Introduction

The Universal Tween Engine enables the interpolation of every attribute from any object in any Java project (being Swing, SWT, OpenGL or even Console-based). Implement the TweenAccessor interface, register it to the engine, and animate anything you want!

In one line, send your objects to another position (here x=20 and y=30), with a smooth elastic transition, during 1 second).

// Arguments are (1) the target, (2) the type of interpolation, 
// and (3) the duration in seconds. Additional methods specify  
// the target values, and the easing function. 

Tween.to(mySprite, Type.POSITION_XY, 1.0f).target(20, 30).ease(Elastic.INOUT);

// Possibilities are:

Tween.to(...); // interpolates from the current values to the targets
Tween.from(...); // interpolates from the given values to the current ones
Tween.set(...); // apply the target values without animation (useful with a delay)
Tween.call(...); // calls a method (useful with a delay)

// Current options are:

myTween.delay(0.5f);
myTween.repeat(2, 0.5f);
myTween.repeatYoyo(2, 0.5f);
myTween.pause();
myTween.resume();
myTween.setCallback(callback);
myTween.setCallbackTriggers(flags);
myTween.setUserData(obj);

// You can of course chain everything:

Tween.to(...).delay(1.0f).repeat(2, 0.5f).start(myManager);

// Moreover, slow-motion, fast-motion and reverse play is easy,
// you just need to change the speed of the update:

myManager.update(delta * speed);

Create some powerful animation sequences!

Timeline.createSequence()
    // First, set all objects to their initial positions
    .push(Tween.set(...))
    .push(Tween.set(...))
    .push(Tween.set(...))

    // Wait 1s
    .pushPause(1.0f)

    // Move the objects around, one after the other
    .push(Tween.to(...))
    .push(Tween.to(...))
    .push(Tween.to(...))

    // Then, move the objects around at the same time
    .beginParallel()
        .push(Tween.to(...))
        .push(Tween.to(...))
        .push(Tween.to(...))
    .end()

    // And repeat the whole sequence 2 times
    // with a 0.5s pause between each iteration
    .repeatYoyo(2, 0.5f)

    // Let's go!
    .start(myManager);

You can also quickly create timers:

Tween.call(myCallback).delay(3000).start(myManager);

Main features are:

Get started and documentation index

Detailed documentation with code snippets and examples is available for the following topics:

Where can I ask for help?

There is a dedicated forum for you: http://www.aurelienribon.com/forum/viewforum.php?f=5

Also, the following link will guide you to a discussion thread that started it all:
http://www.badlogicgames.com/forum/viewtopic.php?f=17&t=494