how to create iphone's wobbling icon effect?

Simple way to do it:

#define RADIANS(degrees) (((degrees) * M_PI) / 180.0)

CGAffineTransform leftWobble = CGAffineTransformRotate(CGAffineTransformIdentity, RADIANS(-5.0));
CGAffineTransform rightWobble = CGAffineTransformRotate(CGAffineTransformIdentity, RADIANS(5.0));

itemView.transform = leftWobble;  // starting point

[UIView beginAnimations:@"wobble" context:itemView];
[UIView setAnimationRepeatAutoreverses:YES]; // important
[UIView setAnimationRepeatCount:10];
[UIView setAnimationDuration:0.25];
[UIView setAnimationDelegate:self];
[UIView setAnimationDidStopSelector:@selector(wobbleEnded:finished:context:)];

itemView.transform = rightWobble; // end here & auto-reverse

[UIView commitAnimations];

...

- (void) wobbleEnded:(NSString *)animationID finished:(NSNumber *)finished context:(void *)context 
{
     if ([finished boolValue]) {
        UIView* item = (UIView *)context;
        item.transform = CGAffineTransformIdentity;
     }
}

Probably have to play with timing and angles but this should get you started.

EDIT: I edited the response to add code to put the item back in its original state when done. Also, note that you can use the beginAnimations context value to pass along anything to the start/stop methods. In this case it's the wobbling object itself so you don't have to rely on specific ivars and the method can be used for any generic UIView-based object (i.e. text labels, images, etc.)

You should use CAKeyframeAnimation to make a smoother animation.

+ (void) animationKeyFramed: (CALayer *) layer 
                   delegate: (id) object
              forKey: (NSString *) key {

    CAKeyframeAnimation *animation;
    animation = [CAKeyframeAnimation animationWithKeyPath:@"transform.rotation.z"];
    animation.duration = 0.4;
    animation.cumulative = YES;
    animation.repeatCount = 2;
    animation.values = [NSArray arrayWithObjects:
            [NSNumber numberWithFloat: 0.0], 
            [NSNumber numberWithFloat: RADIANS(-9.0)], 
            [NSNumber numberWithFloat: 0.0],
            [NSNumber numberWithFloat: RADIANS(9.0)],
            [NSNumber numberWithFloat: 0.0], nil];
    animation.fillMode = kCAFillModeForwards;
    animation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionLinear];
    animation.removedOnCompletion = NO;
    animation.delegate = object;

    [layer addAnimation:animation forKey:key];
}

The easiest way I know is to use Core Animation. Basically, you create an Core Animation Block, then do an rotation transform and setup and repeat count. Core Animation then takes care of everything that's needed to do this wobbling effect.

To start an Core Animation block, just do:

[UIView beginAnimations:@"any string as animationID" context:self];
[UIView setAnimationRepeatCount:10];
// rotate 
[UIView commitAnimations];

not tested. But it can be that you will also have to do:

[UIView setAnimationBeginsFromCurrentState:YES];

A.F.A.I.K. setAnimationRepeatCount will have the effect that the animation gets done, undone, done, undone, done, undone, done... as many times as you specify. So you may want to first rotate to left with no repeat count, and then from this point start wobbling with repeat count. When done, you may want to rotate back to the identity transform (= no rotation and scaling applied).

You can chain animations by setting the animation delegate with

[UIView setAnimationDelegate:self]

and then

[UIView setAnimationDidStopSelector:@selector(myMethod:finished:context:)];

and as soon as the animation stops, that method will be called. See the UIView class documentation for how to implement that method that will be called when the animation stops. Basically, inside that method you would perform the next step (i.e. rotating back, or anything else), with an new animation block but same context and animation ID, and then (if needed) specify another didStopSelector.

UPDATE:

You may want to check out:

[UIView setAnimationRepeatAutoreverses:YES];

this will wobble back and forth automatically.

from above answers, I got the swift5 version:

  func startWobble() {
    let angle = 5.0 * Double.pi / 180.0;
    self.transform = CGAffineTransform.identity.rotated(by: CGFloat(-angle));
    UIView.animate(withDuration: 0.25, delay: 0, options: [.allowUserInteraction,.repeat,.autoreverse], animations: {
         self.transform = CGAffineTransform.identity.rotated(by: CGFloat(angle));
    }, completion: nil)

}

func stopWobble() {

    UIView.animate(withDuration: 0.25, delay: 0, options: [.allowUserInteraction,.beginFromCurrentState,.curveLinear], animations: {
        self.transform = CGAffineTransform.identity;
    }, completion: nil)
}

Easiest way to this with Swift 4+ :

static func wobbleAnimation(button: UIButton) {
    CATransaction.begin()
    let rotateAnimation = CABasicAnimation(keyPath: "transform.rotation")
    rotateAnimation.autoreverses = true
    rotateAnimation.repeatCount = Float.greatestFiniteMagnitude
    rotateAnimation.fromValue = CGFloat(-0.2)
    rotateAnimation.toValue = CGFloat(0.2)
    rotateAnimation.duration = 0.20
    button.layer.add(rotateAnimation, forKey: nil)
    CATransaction.commit()
}

static func stopWobbleAnimation(button: UIButton) {
    button.layer.removeAllAnimations()
}