Someone has already posted the original heartbleed test website, and Filippo is awesome for open sourcing it so you should support him. We actually use part of his test code to power our site.
I worked with a team to setup a site that has a few more features than just seeing if the website is currently vulnerable. We stored the public key of all of the SSL certs of the top 1 million websites and are checking to see that they are changing their certs as well as if they are still vulnerable to heart bleed. We also have some stats on how many sites that we've tested are still vulnerable. Currently it's about 9%.
As like with Filippo's website you can test your own site to see if it's vulnerable. It is heartbleedstatus.com
(Discosure as stated above I was on the team to create this site.)