The paladin is one of the most romanticized, memorable figures in fantasy gaming.  He lives by a strict code of behavior, and follows those principles no matter what.  In exchange, he has special powers and abilities granted to him for use in his neverending battles against evil.

In many D&D games, the paladin is simply an agent of good and law.  While that definition works just fine for those games, the rules allow that a paladin, just as a cleric, can also belong to a specific mythos, or worship a specific deity.  In fact, many seasoned DMs will require a player of either a paladin or a cleric to pick a patron deity.  It is in these games that the question may arise, "why are paladins limited to the gods of good?"

Indeed, is there any non-artifical reason (i.e. just because the rules say so) that neutral (or even evil) deities would not also employ paladins?  To the contrary, I personally believe that it makes perfect sense - if a paladin is a holy knight in the service of his deity (and not of some abstracted sense of "goodness"), then it seems preposterous that non-good deities would not choose to make use of a paladin's services.

This is, of course, a completely optional house rule, but in practice, I have found it to work well, regardless of system (d20, 4th edition, 2nd edition, you name it).  As a matter of fact, one of the most memorable paladins from my personal games was a follower of Shinara, the goddess of luck (from the Audalis campaign setting).  In keeping with the will of his goddess, he would make decisions based upon gut feelings at the time, or would even consult a coin flip for particularly vexing choices.  Caution and planning were for weaklings and cowards; his goddess would either see him through, or she would not.  It was that simple.  As you might guess, this led to many an entertaining situation!

If you follow these guidelines, you can open up a whole new realm of possibilities to your players (and to your campaign world).  Naturally, some deities do not lend themselves well to having paladins (a god of peace, for example, might not use them at all, and if so, in a very limited capacity), but for the most part, it makes sense that almost any deity would certainly take advantage of dedicated holy warriors to their cause. 

Of course, if you do choose to allow paladins of other deities, you must carefully consider alignment requirements (since lawful good will certainly not make sense for, say, followers of a goddess of pain).  For example, if a player wishes to play a paladin of Shinara, I only allow them to be chaotic neutral.  For the most part, however, you should probably limit paladin alignments to lawful varities, unless the deity simply begs for something else.

Next, you must define the code by which the paladin must live.  If your deities are well-developed, this should be fairly simple - the paladin must act in a manner that supports and furthers their patron's ethos.  This code must be every bit as strict and binding as that of the traditional paladin - unintentional transgressions against the code result in temporary loss of paladin abilities until atonement can be done, intentional transgressions result in permanent loss of paladin status.  If, for example, our paladin of Shinara were to take extra precautions (and thus NOT leave his life in the hands of his goddess), he would be transgressing, and subject to punishment.

Finally, you must adjust the granted powers of the paladin accordingly.  Detect good does not make sense as a granted power to an evil paladin, for example.  You must carefully substitute abilities of equivalent power and scope.  You may wish to discuss this with the paladin's player at character creation; this is an inexact science, and may require you, as the DM, to make future adjustments for proper game balance.  Make sure that your player is on board with the idea.

The traditional definition of the paladin works perfectly fine for most games.  However, this optional rule allows for even more flexibility (and in my opinion, realism) for your campaign world.  Happy gaming!