1) Inhabitation:
By conjuring a free spirit of any force (the lower the easier), or by taking the Ally Conjuration metamagic (and spending a whopping 8 karma and 500

Now, since the spirit itself isn't making the attack, the rules become a little interesting at this point. If the spirit made the attack, the weapon would gain a +4 DV and all damage would become elemental in nature. If the weapon is attacked, the attacker has to save vs. a DV equal to the spirit's force (elemental damage as well). By a strict reading of the rules, the weapon would in fact gain no damage bonus; it would simply allow mundanes to attack spirits/manifested/astral forms and ignore their ItNW.
(Disclaimer!: House Rule Ahead!) I believe that if the spirit so inhabiting had the Energy Aura power, a safe, non-game-breaking compromise would be to allow the spirit's force to be added onto the weapon damage (Elemental effects as normal). This, however, is not needed. (/House Rule)
Theorhetically, you could simulate a lightsaber (or like weapon) under similar conditions. Inhabitation spirit with Elemental Aura (Any, Light would be fitting) and Natural Weapon inhabiting a metallic cylinder/sword hilt. The spirit manifests (not like astral manifestation, more like Spirit Mask) its Natural Weapon as a beam of energy, and whammo!
This would also be able to simulate various intelligent weapons throughout myth. The spirit can only talk through the weapon (to whomever is wielding the blade), ect.
The same technique can be used to gain the use of certain powers under certain situations (and to simulate items of significant power as seen in lore and myth), simply by causing an inhabitation spirit with certain powers to inhabit various objects. To make the deal more palatable for some casters, you could promise the freedom of the spirit after a number of tasks have been completed (either the favors owed by the Binding test, or a new number after forcing the spirit to inhabit the object).
Example: Bob, the wise-cracking skull in the Dresden files can be best surmised as an Inhabitation Guidance spirit of force 8+ taking residence in a human skull.
The above concepts are best implemented by mystic adepts; summoning, binding, and "enchanting" their own weapons and equipment.
2) Free Spirits:
This section is mainly for a discussion of free spirit PCs. Through my reading of free spirit rules in Runner's Companion, it would appear the game designers have something against the use of free spirit PCs.
The average 400 BP free spirit PC would be able to replicate roughly a force 2.5 spirit (meaning he could have all the stats of a force 2 spirit, with slightly higher force and some additional skills/abilities), whereas Street Magic tells us that free spirits of such, /ahem, delicate nature are very rare (from a bound - to - free status: most bound spirits that become free are F6+).
Not only that, but the power selection seems more like punishment than an option list. A standard bound spirit going free retains all their powers, and gains more depending on their new Edge, as opposed to free spirit PCs gaining "Spirit Points" (similar to adept's power points) equal to their Edge, and the average power costs 1.6 (rounded up).
I guess the "power" available from playing a free spirit helps mitigate the hate directed towards them, but honestly, if you've only got ~11 boxes in either of your condition monitors, you've got to expect to get disrupted almost every 'run, unless you go for the Concealment power and hope you aren't even seen. On the bright side, you're almost impossible to permanently banish (
Upon writing this, it occurs to me I wouldn't have such a problem with free spirit PCs if they had instead called them Wild Spirit PCs, since wild spirits follow so many fewer of the "known rules" of spirits.
3) Theorhetical Application of Above:
What exactly would happen if an initiated mage with the Ally Conjuration metamagic summoned an inhabitation spirit into himself? How exactly would the rules cover this? More specifically, if he were to arrange the inhabitation to cause a Hybrid Form inhabitation merge. Would the mage have just found a (quasi) cheaper version of playing a more powerful free spirit (and follow the free spirit rules in Street Magic)? The core rules seem to point towards the PC losing control of the character, yet there is some wording within the Inhabitation power that may allow lenient DMs to let the PC retain his character, with appropriate RP changes.
End
So have I missed some things, or are the above more or less true? Discuss.
EDIT: Edited for final thougts and finishing touches. Also revised finishing thoughts for the debate from later pages.
EDIT: Added by me (Neraph), on 11/28/09 from post on: Nov 22 2009, 11:55 PM
4) Additional Information
In Running Wild, page 174, they have added very interesting rules for calling free/wild spirits and gaining favors from them, similar to summoning and binding (in that you can cause a spirit to appear and gain a number of favors from said spirit), under the Calling and Offering section.
The following is a condensed section of pertinent rules:
[ Spoiler ]
5) Applicable Result/Guide
After completing an Occult Knowledge Test, you gain the information to Call a special type of spirit, with the powers listed above. You either craft yourself or have a contact craft for you the paraphernalia required to Call the spirit at Force 1 (two radical animal blood reagents, availability 8, 400


With their 2 dicepool against your Charisma + Negotiation (+ all those extra bonuses we all know and love), you will win.
The spirit (an apparent variant of a blood spirit) at Force 1 cannot gain a True Form Inhabitation Merge, as he cannot exceed the Object Resistance of the weapon you have chosen. This will result in either a Hybrid Form Inhabitation Merge (which is most desirable), or likely a Flesh Form Inhabitation Merge, causing the weapon to become Dual-Natured (now dangerous against those pesky astral forms) and much harder (Immunity [Normal Weapons]), among other things. Please note that this must take place inside a magical lodge with a Force equal or higher than that of the spirit, in this case, 1. Also, the only forseeable problem is that of the spirit in question Critically glitching on the Inhabitation check; this can be fixed with the spirit initially having the Guard power active on itself.
I present to you: Bloodmourne.
Alternatively (and more expensively), you can follow this same preceedure to similarly "enchant" barbed arrows (or bolts). As the arrow is stuck in the enemy, the spirit within begins to deal additional "DoT" (Damamge over Time) damage to him, gaining 1 point of karma per point of physical damage dealt (as per Energy Drain [Karma]). If the enemy wants to stop this, he must remove the barbed arrow.
I present to you: the Blooddrinker Arrows (I brainstormed these with Ragewind, and he insists they should be named Souldrinker Arrows, but as I'm posting this

A 1-Up for Blooddrinker Arrows would be using Injection Arrows as the base arrow, and remember to include a dose of Slab. As they lie there for at least an hour due to the Slab drug, your Bolt-Buddy finishes killing them to suck the karma out of their blood.
In all cases, the Bloodmourne system of Dual-Natured weapons manufacturing creates an extremely flexible system of creating a smart-weapon capable of aiding its wielder immensely (and possibly similarly to using mooks for hacking), and it only gets better over time.