Lol dude, Mods are usually for single player. You never see mods for multiplayer. The only modifications you can really do to multiplayer are considered plugins, and they do have a Herobrine plugin if you actually search around.

^WroNG

You need to download the server side of the mod (mod plugins like buildcraft have server sided)

then download minecraft_server from minecraft.net.
then right click minecraft_server and click open with Win Rar. Then drag and drop your mods into the server file (or follow mod instructions)

Dont know how im wrong. Herobrine server side plugins? They dont have those. Thats what hes asking. Do they have it, no. So hes gonna have to turn over to plugins by bukkit. And please if you know where to find the server side plugin for herobrine please feel free to speak up and help this guy out, rather than try to prove me wrong, as you dont know what your talking about when it comes to a herobrine server-side plugin.