View floating above all ViewControllers

There is. You can add your view to the main window and bring it to front when you have to.

In following code is presumed that _viewConroller and _anotherView are strong properties of appDelegate - configuration could of course be different.

This code would add small blue square on top left corner of the screen.

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

    _viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    _anotherView = [[UIView alloc] initWithFrame: CGRectMake (0.0,0.0,20.0,20.0)];
    [anotherView setBackgroundColor: [UIColor blueColor]];    

    self.window.rootViewController = self.viewController;
    [self.window makeKeyAndVisible];

    [self.window addSubView: _anotherView];
    [self.window bringSubViewToFront: _anotherView]; //not really needed here but it doesn't do any harm

    return YES;
}

You can do the following if you are using storyboard and autolayout (inspired by the first Answer)

UIStoryboard *sb = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
UIViewController *_vc = [sb instantiateViewControllerWithIdentifier:@"FloatingController"];

_anotherView = _vc.view;
[_anotherView setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.window addSubview: _anotherView];

[[VLBConstraintsGenerator sharedInstance] setWidth:200 forView:_anotherView inSuperView:_window];
[[VLBConstraintsGenerator sharedInstance] setHeight:200 forView:_anotherView inSuperView:_window];
[[VLBConstraintsGenerator sharedInstance] setLeading:0 forView:_anotherView inSuperView:_window];


[_anotherView setBackgroundColor:[UIColor grayColor]];

[[_anotherView layer] setBorderWidth:1];
[[_anotherView layer] setBorderColor:[UIColor yellowColor].CGColor];

[self.window makeKeyAndVisible];
[self.window bringSubviewToFront:_anotherView]; //not really needed here but it doesn't do any harm

all you need to do is to drag a view controller into you main storyboard with FloatingController as a storyboard ID

Additional Methods

-(void)setWidth:(CGFloat )theWidth forView:(UIView *)theView inSuperView:(UIView *)theSuperView

{
 assert([theSuperView isEqual:theView.superview]);
    NSLayoutConstraint *cn = [NSLayoutConstraint constraintWithItem:theView
                                                          attribute:NSLayoutAttributeWidth
                                                          relatedBy:NSLayoutRelationEqual
                                                             toItem:nil
                                                          attribute:NSLayoutAttributeNotAnAttribute
                                                         multiplier:1
                                                           constant:theWidth];


//    [cn setPriority:999];//make it variable according to the orientation
[theSuperView addConstraint:cn];
}


-(void)setHeight:(CGFloat )theHeight forView:(UIView *)theView inSuperView:(UIView *)theSuperView
{
assert([theSuperView isEqual:theView.superview]);

NSLayoutConstraint *cn = [NSLayoutConstraint constraintWithItem:theView
                                                      attribute:NSLayoutAttributeHeight
                                                      relatedBy:NSLayoutRelationEqual
                                                         toItem:nil
                                                      attribute:NSLayoutAttributeNotAnAttribute
                                                     multiplier:1
                                                       constant:theHeight];

[theSuperView addConstraint:cn];
}

-(void)setLeading:(CGFloat )theLeading forView:(UIView *)theView inSuperView:(UIView *)theSuperView
{
assert([theSuperView isEqual:theView.superview]);

NSLayoutConstraint *cn = [NSLayoutConstraint constraintWithItem:theView
                                                      attribute:NSLayoutAttributeLeading
                                                      relatedBy:NSLayoutRelationEqual
                                                         toItem:theSuperView
                                                      attribute:NSLayoutAttributeLeading
                                                     multiplier:1
                                                       constant:theLeading];

[theSuperView addConstraint:cn];
}