This is not a post about what is HeartBleed and how "evil" it is, I'm assuming you already heard about it ...
Anyway, you should find all the details at http://heartbleed.com/
Here is a collection of nice tools to play with:
- website to test online a server against heartbleed exploit: https://filippo.io/Heartbleed/
- script to simulate a heartbleed attack: https://gist.github.com/takeshixx/10107280
- server to test the script against (if you don't have one already): heartbleed.insign.ch:443
- iptables rule to drop all heartbeat requests: http://www.securityfocus.com/archive/1/531779