Keep some chunks loaded in Minecraft SMP

I am looking for a mod or something which would allow me to keep some chunks loaded on the SMP server I administrate.

In fact, it is a redstone circuit including a clock under a huge mob trap which triggers some pistons and water bringing loots regularly to a point. This would allow mobs to spawn continuously and the redstone circuit also to run without having to be reset when we want it to work (because the power freezes when the chunks are unloaded and doesn't restart afterwards).

Does something like that exist ?

Minecraft 1.13.1 and newer versions have a /forceload command:

/forceload add x z

Where x and z are the coordinates of the chunk you want to load (specified as number of chunks from 0,0), not the number of blocks

If you're using Bukkit you can use Forever ALoad, where you can select regions of map to keep permanently loaded.

Once installed it has a very simple chat interface:

/chunk        - Tells you how many chunks you will have (at a maximum) 
                remaining from unloading, and give stats about it

/chunk add    - Adds the chunk you're on to the Do-Not-Unload list

/chunk remove - Removes the chunk from the list

/chunk check  - Checks whether the chunk can unload