Creatures that can't cast spells use disks for buffs
The 6th-level Sor/Wiz spell Shalantha's delicate disk [conj] (Lost Empires of Faerûn 33) compares favorably with other methods of casting upon the creature that ruins the disk spell effect a spell that the creature couldn't normally benefit from.
For example, a potion of divine power (4th-level spell at caster level 7) (1,400 gp; 0.1 lbs.) is normally impossible to create, both because the spell is higher than a 3rd-level spell and because of the divine power spell's entry of Range: Personal, yet the disk spell's effect can hold a divine power spell and cast the divine power spell upon the creature that ruins the disk spell effect. (Some prestige classes can certainly create 4th-level and higher potions or their equivalent, but none so far as I'm aware overcome the limitation that a spell in a potion can't have an entry of Range: Personal.) (Also see for Pathfinder this question that applies equally to D&D 3.5.)
While it can be argued that if such creatures want spells then they should take ranks in the skill Use Magic Device and buy scrolls or whatever, the Use Magic Device skill isn't for everyone, and sometimes when using monsters straight from the book they need a boost. The disk spell, for them, is awesome. For example, the typical pit fiend (Monster Manual 57–8), charming as it is with its Charisma 26, has no ranks in the skill Use Magic Device. (It also has only 2 ranks in the skill Balance. Tsk tsk.)
Dumpster-diving for disk spell alternatives yields few results.
The feat Attune Gem (Magic of Faerûn 21) certainly does come close but the magic items it creates haven't aged well, dating back as they do to the game's early days and fraught as they are with ambiguity.
Closer to the disk spell's versatility is the feat Craft Wondrous Item (Player's Handbook 92–3) when used to create a spellworm (Dragon #343 80–1), but a spellworm's creator either must possess the distinction wormspawn (80) or—probably!—possess the feat Wormbound (76). (There's also the fact that each time a creature opts to let a spellworm burrow into it that the creature will fail a Will save (DC 20) and join forever an apocalyptic death cult. So there's that.)
Also, creating a magic item typically takes both at least 1 day and at least some XP, but casting a disk spell takes only a standard action, spell slots, and gp. This makes it so that if disk spell effects are readily available they'll possibly replace a number of 1-use items.
Finally, while a disk spell effect can't hold a spell higher than 5th-level, and a disk spell effect's duration is merely permanent rather than instantaneous, the price of a magic item usually increases based on the caster level, and that's just not so with the spell stored in a disk spell effect: the power is as impressive as the savings are substantial. (An example of this math is below.)
It was this last that eventually stopped me as a DM from using the disk spell.
In campaigns that I run—that see fairly optimized PCs—I started equipping NPCs with disk spell effects that they'd break whenever the NPCs had the opportunity to buff themselves.
First, there's the option paralysis that comes from having available a tremendous number of spells, on the same scale of equipping NPCs with partially charged wands (see answers to this question).
Second, it was too easy for me as the DM to justify NPCs having an arbitrary number of buff spells on them all the time that the PCs couldn't prevent them from having on them because the NPCs buffed themselves at their adventuring day's start. Subtracting the price of the spellcasting service from their NPC wealth seemed to make things even more unfair: NPCs put up a much tougher fight but had less gear when defeated! For example, the price of a level 11 caster's disk spell is 860 gp, and the price of a level 20 caster's divine power spell is a mere 800 gp. With disk spell effects having such a small price tag, they were too good to ignore… so I had to ignore the spell—essentially banning it from my NPCs' gear lists—lest all encounters turn into the same encounter.
I don't ban the disk spell from PC use, but I do warn players to use the spell cautiously: The disk spell's price per casting is too high to necessitate it absolutely, but the spell's very nearly something around which I must design a campaign. (If you're interested, other game elements that I think require a campaign redesign are in answers to this question and this question.) When it makes sense, I'll still equip NPCs with a disk spell effect, but that's very rare. (In the campaign I'm running now, the PCs have advanced from levels 1 to 16 and haven't encountered an NPC toting a disk spell effect.)
Note: The power of the Shalantha's delicate disk spell is lessened significantly—but by no means obviated!—if the spell effect can't store spells with an entry of Range: Personal. That is, the spell's description says that when the disk effect is shattered that "the spell immediately takes effect as if it had just been cast by the spellcaster who first placed it in the disk." Then it goes on to say that "[t]ouch-range spells and spells that have specific targets… are targeted upon the creature or object that shattered the disk," but then it later uses as an example of a valid spell the flame blade spell… that lacks a Target entry completely yet that causes a "3-foot-long, blazing beam of red-hot fire [to] spring… forth from your hand." In sum, this DM reads the Shalantha's delicate disk spell broadly—as he believes he should with a spell that's this high a level yet somewhat ambiguous—, but another DM may read the spell somewhat more or even far more narrowly therefore finding it not nearly as problematic.