TestiPhone.com, a clever web-based tool for testing content as it would appear on the iPhone. It's intended use was to test iPhone web apps prior to the advent of native apps. Now that the SDK is out, which provides a full blown simulator (with some limitation), this free online tool would seem to be obsolete. But I think it and others like it (iPhoney: iPhone web simulator) can still be of service for browser compatibility testing. With iPhones accounting for most of the US mobile browser web traffic, website developers need to pay attention to how their pages look on mobile Safari. Hence testers need to add mobile Safari to the browser compatibility test matrix. And by having a tool that simulates the iPhone running on the same system with desktop browsers, you can view web page rendering side-by-side. And best of all it doesn't require installation of the iPhone SDK.
Certainly you'd want to do at least one pass on an actual iPhone, but during most of the development cycle, having a quick and easy way to check how your website looks on mobile Safari, makes these simulators worth adding to your tester toolbox.