It was leaked on Reddit, but later Notch said that it was OK that people used it. Jeb, a Minecraft Developer was most likely the one that leaked it, he said that he would release it if he solved a problem for "expert users".
Here's the link: http://assets.minecraft.net/1_8-pre/minecraft_server.jar
I didn't download it (because I don't have a Minecraft account lol) and I don't have a computer good enough to run it, but you can have fun with it!