In the first edition DMG there are more than 2 pages devoted to sages in how much it costs to hire them, what kinds of magic they have and what fields of knowledge they specialize in. What it doesn't say is what are the limits of the questions that can be asked of them. For many years, I have considered them just encyclopedias, effectively hooks to send the party out to some ruin or distant city to find some item, person or material. 2-3 years ago, Old Geezer on the rpg.net's forum (one of EGG's players) mentioned something that really opened my eyes on the use of sages. I don't remember the specifics, but it was something powerful. In effect, a sage can answer a question that can alter the nature of a character. Obviously they can do more than that and I have been ruminating on the use of sages since then. Here are some questions that I have come up with that cover a lot of territory. They include what classes are most likely to ask the questions (I completely ignore the limitations of classes who can hire sages). The ones that druids might ask are usually not to human sages, but rather elemental wierds (the 3e version), spirits of the land or other nature spirits are the source of information. Sages that clerics go to may be angels or demons or just special people within the faith. All of them have many different answers that work and I provide a few of my own. Each answer has its own price (i.e. the cash the book mentions).

(Any) How can I learn to fly? The character may learn how to create hot air balloons, get wings grafting to them by xixchil, get infused with air elemental energies or ride a gryphon.

(Any) How can I discover those who plot against me? The character may have their eyes replaced with those of some creature with clairvoyance, may quest for a special crystal ball or convince the spirits of the kingdom to help dig out traitors.

(Any) How can I absorb the strength of my foes? Rituals that allow the character to improve their ability scores, magical abilities or gain innate spells are the basic way for this. They are quite costly and do weaken the foe. Of course the foe can use a similar ritual to get their power back.

(MU/I) How can I use my magic to enhance this wand? The character may under go an expensive ritual that burns one of their high level slots permanently so that they can apply a single metamagic spell to the wand or they may drink a potion of dragons' blood (all 5 chromatics) which allows them to use any metamagic spell on the wand (the potion must be used per casting).

(FRP) What ritual must I use to turn this blade into a dragonslayer? The warrior may have to collect dragon eggs and then have their magic-user ally infuse their strength into the sword, they may have to find a dragon eating monster and have it bless the blade or they may have to take the weapon to another plane, one hostile to dragons and plunge it into a magical pool there.

(D) How do I infuse air elementals into trees? To create a sort of golem creature, the druid may have to ask djinn lords for their assistance, ask efreeti lords for their assistance, use elemental air as bait or learn a special version of Reincarnate that produces elemental creatures and use it on an intelligent creature, hoping to get an air elemental and then using a new 7th level spell to fuse it with the tree in question.

(Any) How do I become an earth elemental. Who says liches are the only way to immortality? One could use a basilisk that has been exposed to energy from the Plane of Mineral or use a soul ripping divine spell to possess an existing earth elemental.

(C) How do I close this gate to Hell? Self sacrifice, using a mace blessed by the god himself or stopping time at the entrance are all possible answers.

(D/MU) What might happen if I slay and reincarnate the hill giant chief? If the form is useful, it may cause chaos throughout the hill giant clans and cause a civil war or it may cause the giants to look upon the caster as a god that smites foes in a horrific way, causing them to abandon the region for a few generations.

(C) How do I feed and care for all the refugees from the colony? Use the temple's funds to hire the local people to assist, summon a (specific) deva that can do such or build a new quarter for the city and have them assist themselves.

(D) What elementals must I summon to clean up a (specific) wasteland? It all depends on the source of the destruction, so I (as in me, not the sage) can't answer that.

(Any) Where are the remains of my clan's founder and how can I use to to become the new leader? I can't answer to where as that is setting specific, but the spirit of the found could be used as an advisor, speaking from their skull or their memories could be infused into the character.

As you can see, the sky is the limit for the potential for the kinds of information that a sage can give. The time that it takes for a sage to come up with the answer allows the DM to have some breathing room and there is always the option of "No, that isn't possible." That or make it so costly that only the most dedicated or boneheaded characters will attempt it.

Note that in 3rd edition, this could be a way to discover or develop new feats.