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The jar can be downloaded from https://s3.amazonaws.com/Minecraft.Download/versions/1.8-pre3/minecraft_server.1.8-pre3.jar

As for running it it can be done through a control panel like Multicraft, or if you are hosting it on your computer create a file called start.bat and inside it have this text below.

java -Xms1024M -Xmx2048M -jar minecraft_server.1.8-pre3.jar

The 1024 is the minimum ram, and the 2048 is max ram, both in MB, and both can be changed to whatever you want.

Change minecraft_server.1.8-pre3.jar in that scrip to whatever the server jar is called in future updates.

More in depth information can be found at http://minecraft.gamepedia.com/Tutorials/Setting_up_a_server