The Rules: The feat, Sanctum Spell, can be found in Complete Arcane. In short, it allows you to define(and occasionally change) a sanctum for yourself. It’s a +0 metamagic feat, which allows spells prepared with it to be cast at +1 spell level if in the sanctum, and -1 spell level if outside it. They take the usual slot, but for EVERYTHING else, they use the modified spell level.
The Intent: Almost certainly, designer intent was for this to be an option for a defensive character, with the modified spell level mostly affecting save DCs. Unfortunately, PCs tend to be active, often adventuring in different locales, and spend only a minority of their time in a single building. Thus, it’s a quite poor feat for it’s original purpose.
The Trick: A great many things key off spell level, and since this feat can adjust them either way, it’s immensely useful for everything but it’s intended purpose. Want to get a level 5 spell on a wand? Sanctum Spell. Want to get into a casting PrC two levels early? Sanctum Spell. Combined with Divine Metamagic, this allows very early entry indeed to Mystic Theurge, will save you a bunch of money on utility spell scroll/wand crafting, and is generally a great way to spend an extra feat.
DM Info: Powerful as the trick is, it’s surprisingly balanced most of the time. Cheaper wands and scrolls, for example, are balanced by having a lower DC. This makes the trick only really useful for saving a few gold and xp on crafting scrolls of utility spells to help the party, and frankly, crafter wizards usually lag behind the party in gold and xp anyway, and are not terribly broken builds. Likewise, dual casting PrCs like Mystic Theurge are usually held in low regard among optimizers due to getting spells vastly behind regular casters. In short, most uses of this trick result in making otherwise poor options competitive.
What you should never do with this trick: Apply Sanctum Spell to a cantrip, then craft a wand of it, outside your sanctum. It’s now a -1/2 level spell. CL is, as always, a minimum of 1. Therefore, you EARN money and xp for crafting wands. Use of the trick in this manner is sufficiently cheesy to make your DM quite justified in beating you about the ears with the heaviest rulebook available.