Introduction:

"Heartbleed is a security bug in the open-source OpenSSL cryptography library, which is widely used to implement the Internet's Transport Layer Security (TLS) protocol. This vulnerability, classified as a buffer over-read, results from a missing bounds check in the handling of the Transport Layer Security (TLS) heartbeat extension, the heartbeat being behind the bug's name."
The explanation of heartbleed taken from https://en.wikipedia.org/wiki/Heartbleed.

In other words Heartbleed is a vulnerability that pretty much allows you to obtain data being sent allowing you to obtain information like usernames,passwords,emails,etc.

Which Sites Are Vulnerable:
When this exploit first came out (April 3, 2014) every site using SSL was vulnerable to this causing a very big problem for even the biggest of sites. At 1 point sites like google,tumblr,etc were vulnerable but most big sites have been patched a hour or a day into the discovery.

Due to this being a old exploit it is mostly patched on all SSL sites but can still be abused on 100's of websites.

These SSL version are at the moment vulnerable and patched:
OpenSSL 1.0.1 through 1.0.1f (inclusive) are vulnerable
OpenSSL 1.0.1g is NOT vulnerable
OpenSSL 1.0.0 branch is NOT vulnerable
OpenSSL 0.9.8 branch is NOT vulnerable

To find a site that is vulnerable download the following plugins on your web browser:
Chrome Bleed: Chrome Bleed Link
Firefox Heartbleed: Firefox Bleed Link

You may also use 1 of these 2 sites:
http://tif.mcafee.com/heartbleedtest
https://filippo.io/Heartbleed/

To find vulnerable websites try using this dork in a google search:
inurl:https://
If you wanna target a certain type of site just type in a keywoard before the dork I provided. For example:
hunting inurl:https://

How To Exploit Sites:
Found a site? Well now lets find out how to exploit it and be rolling in the accounts. First download Bleedout. This program allows you to exploit sites. Put this in a folder as it does create many log files.

Open CMD and type in "CD C:\Users\(Your Computer Username)\Desktop\Bleedout" In other words just do "CD" command and type in the directory bleedout is located in. After type "bleedout" for info on how to use it or just read what I am about to type up. Type in "bleedout -h (domain name without http://)". Then let this run for hours.

After a while check in the folder there will be a text file which will have all the scraped info. Open the text file then press "CTRL + F" and type in password. It will show you some results just look through them and try the logins out. Thats it, you will have a list of accounts soon enough. Ignore the random text that will be generated, this can be used to obtain a SSL private key but either then that they are just replies from the host.

The official site of heartbleed is: http://heartbleed.com/