Trend Micro Heartbleed Detector is a simple Chrome app which can check a website for the Heartbleed vulnerability.
You can run this directly from the link, or "install" it, adding a Trend Micro icon to Chrome's App Launcher.
Click this, and a dialog box presents you with a box asking for the target URL. (There's also a link to "find out more" about Trend Micro's other tools, conveniently. Although it's text only and very unobtrusive - no animated banner ads here.)
Enter the domain, click "Check now", and the verdict is returned within a few seconds.
Close the window if you're done, or just minimise it for more convenient reuse later.
Heartbleed Detector is a simple and apparently effective way to check sites for Heartbleed problems. We're not sure that it's notably better than the other web-based detectors, but the fact that it comes from such a trusted vendor may give you more confidence in the results.