More and more sites are adopting XML syndication technologies such as RSS and ATOM which users can subscribe to.
Rather than being a push technology like email newsletters, RSS is a pull technology. The subscriber is in full control of the subscription, the publisher does not have a relationship with the subscriber, or need to know their email address. This makes unsubscribing very easy and because you don't need to supply an email you do not need to worry whether your details will be sold on by unscrupulous companies.
XML syndication has been around for over 5 years or so, but in the early days the RSS readers available weren't up to scratch, so it took a while for the technology to gather momentum. Nowadays there are plenty of good readers, such as Bloglines, Google Reader etc, which are very polished products that support all the major formats.
The last nail in the email newsletters coffin will be the adoption of RSS advertising into the mainstream. Currently Google and Yahoo! are performing tests with advertising on these syndication formats. As soon as these are released the already strong relationship Google has with publishers will allow it to rapidly make RSS very lucrative for website publishers.
Until recently publishers syndicating their content via RSS had a hard time analysing their circulation, that's where companies such as Feedburner have found a niche and continue to provide publishers with additional services on top of basic subscription tracking.
Of course syndicating your content is just another method of publishing. First you had paper, then HTML now XML. You can't irradiate SPAM with RSS, people can set up SPAM blogs etc, but it's the subscribers who are in control of their subscriptions. So as a publisher you know that your 500 subscribers reported by your RSS analytics product of choice are actively reading your content or else they'd simply click to unsubscribe from within their RSS reader application. Compare that to a database of registered subscribers dating back several years; are those users viewing your newsletter in their preview pane and pressing delete rather than unsubscribing via an unsubscribe link?
Content is King
The old adage that 'content is King' is truer than ever with RSS syndication. The problem with giving such power to the subscriber is that your content needs to be top-notch in order to keep your subscribers subscribing. Even though there are guidelines specifying opt-out and unsubscribe methods and practices, which newsletter senders must adhere to, the fact is unsubscribing from RSS is far easier and is not reliant on differing geographic data protection laws.