Make UINavigationBar transparent

From IOS7 :

self.navigationController.navigationBar.translucent = YES;
self.navigationController.navigationBar.shadowImage = [UIImage new];
self.navigationController.view.backgroundColor = [UIColor clearColor];
[self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
self.navigationController.navigationBar.backgroundColor = [UIColor clearColor];

If anybody is wondering how to achieve this in iOS 7+, here's a solution (iOS 6 compatible too)

In Objective-C

[self.navigationBar setBackgroundImage:[UIImage new]
                         forBarMetrics:UIBarMetricsDefault];
self.navigationBar.shadowImage = [UIImage new];
self.navigationBar.translucent = YES;

In swift 3 (iOS 10)

self.navigationBar.setBackgroundImage(UIImage(), for: .default)
self.navigationBar.shadowImage = UIImage()
self.navigationBar.isTranslucent = true

In swift 2

self.navigationBar.setBackgroundImage(UIImage(), forBarMetrics: .Default)
self.navigationBar.shadowImage = UIImage()
self.navigationBar.translucent = true

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Discussion

Setting translucent to YES on the navigation bar does the trick, due to a behavior discussed in the UINavigationBar documentation. I'll report here the relevant fragment:

If you set this property to YES on a navigation bar with an opaque custom background image, the navigation bar will apply a system opacity less than 1.0 to the image.

In iOS5 you can do this to make the navigation bar transparent:

nav.navigationBar.translucent = YES; // Setting this slides the view up, underneath the nav bar (otherwise it'll appear black)
const float colorMask[6] = {222, 255, 222, 255, 222, 255};
UIImage *img = [[UIImage alloc] init];
UIImage *maskedImage = [UIImage imageWithCGImage: CGImageCreateWithMaskingColors(img.CGImage, colorMask)];

[nav.navigationBar setBackgroundImage:maskedImage forBarMetrics:UIBarMetricsDefault]; 
[img release];