A while ago I read one rather amazing idea- alchemy as spirit magic. Instead of performing magical chemistry, the caster asks the spirit of the material to transform. I like that idea a lot but I don't think that the existing spell system in OSR/d20 does it justice. So after some thinking and reading, I am going to go with incantations, the rules found in Urban Arcana and Unearthed Arcana. They are very easy to modify and allow for non-spell casters to have access to powerful magic. These rules can be used for much more than alchemy. Any sort of magic that the spirits can perform can be used with these suggestions.
For all of them, the spirit's attention must be gained. This can take between a minute and a month, depending on how powerful the spirit is. A spirit of a sword is much weaker than that of an elder oak or river. It requires a Diplomacy check, or reaction roll for OSR games.
The easiest modification is payment. Spirits don't do much but they do have desires. The spirit of a sword may wish to taste the blood of a kind of dragon it has never tasted before, or one of the same species it last killed that is much more powerful than the last. For living things, which can include people, protecting their flesh is basic while protecting their offspring is advanced (and very time consuming). In 3.X, payment with xp is acceptable but that means the spirit can gain levels, an idea that the DM will have to consider. Do something for the spirit and it will perform the action requested. Hopefully.
A shaman could be brought in and make a pact with the spirit (alchemical shamans, weird). This is sort of like payment but has to do with the spirit world, rather than the material plane. Politics of spirits can be confusing to the living and best left to experts. Shamans may also be required to slay or enslave other spirits to please the spirit courted. And that can have as powerful consequences as the desired spell, if not more.
Then there is being bound to the spirit. The spirit wants to ride along with the character in their body or gear or take over entirely. Instead of tasting the blood of a dragon, its spirit wants to be the one who makes the killing strike. Hopefully the spirit will not be addicted to the action or new vessel (ala spirit riders in Farscape).
And then there is the much more involved version of binding. In Atlas' Occult Lore there is a chapter entitled Spirit Cultivation. The gleaner takes the bits of soul left behind after death and gives them power to recieve some, eventually creating new spirtual entities. It is a very cool class, if a bit underpowered. For incantations as spirit magic, this is when the spirit is taken out of its vessel, given a lot of xp, transforms into a different kind of spirit and then is placed back into the vessel. The altered spirit makes changes to the vessel as a side effect (though one desired by the PCs).
After the intial check and the payment is made, then the incantation rules apply. The PCs must go through the ritual and hope the spirit accepts the payment. The more precious the payment, the easier it is to get the spirit to accept. This means much easier DCs. (For OSR games, the checks might be non-weapon proficiency, level checks, saving throws or reaction rolls depending on how the DM want to use it.) If they are included, backlashes should relate to the spirit. So the sword may take a limb or destroy armor, the river may send the PCs out into the sea and the oak may turn the entire forest against them. And those happen on successful checks! Failures tend to be much worse. Spirits are fickle and the incantations should show that.
Obviously this is not combat useful magic, at least outside a siege. But it does allow the PCs very powerful magic with easy controls by the DM. If you don't want the players to move a mountain range, have the spirits tell them off or make the requirement take a thousand years (a pittance for them). It can add a lot of hooks and adventures, something most DMs are happy to get.
