Tom Brennan, the founder of ProactiveRISK created a Mozilla Firefox extension that checks if the websites are vulnerable to Heartbleed bug. The extension is now available at the official Mozilla add-on store. This extension uses the web services developed by Filippo Valsorda where it checks the URL you have just visited.
The browser extension provides color-coding warnings for the websites: Green means it is safe. Red means the website is vulnerable to Heartbleed. Yellow means the website might be vulnerable.
A similar tool has been released for Google Chrome as well. Jamie Hoyle developed the Chromebleed Checker that can also be implemented into your Google Chrome browser for free.
LastPass also released a free website scanner which provides some additional information about the SSL certificate used by the website.