At the moment there is a lot of trouble while downloading some server files (Bukkit for example). I've collected some serverfiles for you, so you can download that files here without ads.
The default server software for big servers. You'll be able to install plugins and so on
An lightweight alternative for Bukkit. You can install Plugins here too
Here you can install Plugins also, but here Forge is installed too, so you can use Mods and Plugins
Vanilla Forge allows you to install Mods on your server. Plugins aren't possible here
This software is written leightweight as Spigot but it doesn't support all Plugins
This is a C#-Written Minecraft-Server, verry verry leightweith. It needs 0.001% CPU Usage without players