The Dimensional Anchor is a block that loads chunks, and keeps them loaded constantly. There is a limit on the number of chunks each non-op player can load in a SMP world, and each anchor can load chunks for an adjustable area of 1x1, 3x3, 5x5, 7x7 or 9x9.
It is more expensive than a Teleport Tether but is able to load more chunks.
ChickenChunks also have a Chunk Loader, which is more easily configurable and about just as expensive.
on the Dimensional Anchor to open the Interface.
If you are in Singleplayer, OP, or the owner of the chunkloader, you can change the size of the loaded area without breaking and replacing it (which would make you the owner). The green button expands the area loaded by this chunkloader by one chunk, and the red button shrinks it, by one chunk. As you can see, there is a limit on the number of chunks that each player may force to be loaded. In singleplayer, or if the owner of the chunkloader is an op, there is no limit.