Chrome: Amazon has released an official Send to Kindle extension for Google Chrome that allows you to send any web pages directly to your Kindle device in one click.
Send to Kindle sits next to Chrome's Omnibar, and when you click it you can send a full web page, or selected text directly to your Kindle (you also get keyboard shortcuts: Alt+K to send, or Alt+P to preview). You can also preview it before you send it out if you want to make sure the extension has properly stripped out all the ads.
The end result that's delivered to your Kindle is a lot like what you'd find with Instapaper or Readability. The extension works with all the different forms Kindle, including the apps for Android and iOS.
Send to Kindle is currently only available for Chrome, but Firefox and Safari extensions are in the works. If you prefer those browsers, check out the bookmarklet Klip.me for similar features.