Morrowind:Command

Makes the target fight for you and follow you for   seconds, if   is greater than or equal to the target's level. There are actually two separate effects: 
Command Creature works on all creatures.
Command Humanoid works on all NPCs.
The afflicted will behave much like a Summoned creature: they will cease any formerly hostile action against you, follow you (including through most forms of fast travel), engage against hostiles, and turn hostile if you hit them 3 times.

Tips
Commanded creatures are ripe for some back stabbing. Just command the creature, deal your stealth hits, and command it again.
Command Humanoid is a good means of bringing NPCs into a different location, as they will follow you, even to different cells, as long as the effect is active.  You can use this to lure a vendor out of their shop and loot the place, lure an intended murder victim away from any potential witnesses, or bring non-freeable slaves into a location where you can free them.
Command spells are also good to just bring service providers to somewhere more convenient:
Want to have your own Guild Guide?  Just Command one of them and bring them to your house.  The destination from the other Guild Guides will stay the same, but it allows you to instantly transport yourself to any of the Mages Guild cities – aside from the one you "hired" the Guide from – right from your house. This is best done with the Guide nearest to your player home; if you take one from a remote location (e.g. Sadrith Mora if you reside in Caldera), the lack of a Guide there any longer will require you to use other means to leave that area.
Want a merchant or enchanter or spellmaker or whatever nearby?  This works on them as well. 
Siltstrider and ship travel providers can (at the risk of breaking believability) also be taken home with you to provide travel far away from the actual means of it. As with Guild Guides, this may be less convenient than it sounds, since they'll no longer be available at their proper location.
Other side effects: Merchants you relocate will only have available what they are carrying (much merchant inventory is usually in containers and on shelves in their shops). NPC dialogue options may change markedly to match their new location – including loss of quest-related options, so choose your new "employees" carefully. The character articles on this wiki indicate what an NPC's quest connections and special dialogue (if any) are.
Another useful Command trick is to use the spell to move NPCs who are blocking doorways or frequently traveled paths. Due to a bug in animation cycles, most "stationary" NPCs slowly migrate forward over the course of the game, and can become obstructions that this spell can periodically rectify.
It can also be used to move siltstrider and ship operators to the ground before they eventually migrate and fall (which can sometimes kill them, either from impact or drowning, respectively).
Commanded targets that respawn (like no-name guards) won't remain where you left them, once the respawn is initiated.
Even a low-duration Command spell can be effective at breaking up large groups of enemies when cast from stealth or long distance.  Enemies that turn against each other will continue fighting even after the spell wears off, provided you don't present yourself as a new target.  This strategy can be useful in the early game when the casting cost of a long-duration version is prohibitive.
The required magnitude of Command for the spell to affect the target is the target's level. For instance, Dagoth Ur and Vivec require a magnitude 100 Command because their levels are 100. Commanding most creatures is rather easy, as for the most part they are very low levels. NPC levels vary widely, but most of those you do not need to fight at some point, and who are not trainers, are low level.
The   ( ) Console command will put Commanded NPCs who are in the same cell with you back in their original locations.

Bugs
 
If you plan on relocating a trainer or a transportation NPC, refrain from using their services until you're positive that the spell is no longer in effect (most easily tested by walking away and seeing if the NPC still follows).
 
If you lose somebody, don't panic.  First, check their default location.  If that doesn't work, take a trip to the Daedric shrine of Assarnatamat (it is dead-center in the map, and the default location for "lost" characters).
 
A permanent Command can be broken by teleporting away from the spell's target and out of the cell. Wait three game days (72 game hours) before returning to the cell. The spell will then no longer be in effect.
 
This seems to happen when their hostility was scripted and not the result of their Fight level. The most common version of this is when one or more NPCs are attacking another (or attacking you, for escorting), and you are supposed to save the victim and kill the aggressor(s); an area-of-effect Command or Calm can simply make everyone stop fighting indefinitely.
This can cause quest-completion problems, when the fight needs to happen but now your target is calm and there are witnesses (even former victims can turn witness in this scenario).  Use Frenzy Humanoid or Taunt to reinitiate combat without a bounty.

Related Effects
Calm, Frenzy
Demoralize, Rally, Turn Undead