How can I play multiplayer against creeps on Minecraft SMP servers?

Solution 1:

A further update: As of Minecraft Release 1.0 (as in, after Alpha and after Beta), SMP offers everything single-player does, including Survival mode, Creative mode, and the Nether and End regions. There are still glitches that only affect SMP, meaning certain animations and so forth might not play properly, but the game is no longer woefully incomplete in SMP.

As of Release 1.3.1, SMP and SSP are now using identical code. Single-player is now a one-person instance of multiplayer. Thus, all differences between the two have been reconciled.

Solution 2:

Add monsters=true (or spawn-monsters=true) to your server.properties file, and monsters will spawn. This is the default setting on minecraft servers; set them to =false instead for no monsters.

Solution 3:

Such things are currently not supported. The game is still in Alpha, which means there are still features to be built and that it is not to be taken as a reliable polished game.

Luckily, there is a rather large update coming October 31st. Here is Notch's blog describing what you can expect from it: http://notch.tumblr.com/post/1243370641

So, for now, Survival Multi Player is more of a sandboxy free-building mode and less of an actual Survival experience, but as the game continues to be developed, SMP will come into its own.

Edit: Clearly this answer is completely outdated.

Solution 4:

Just as an update, with the exception of the Nether the SMP server providers everything SSP offers, including Creepers and the ability to kill them. (Yeah, that way around sounds better... Sssss....)