Since mages (and arcane casters in general) have always been my favorite class, one spell that's always been at the top of my list has been Find Familiar.
It's also a spell that doesn't show up on my rules system of choice: the Rules Cyclopedia. In fact I don't think it shows up until Advanced Dungeons and Dragons, and never makes an appearance in basic/classic/BECMI.
For reference, this is the spell as taken from OSRIC:
Arcane Conjuration/ Summoning
level: Magic user 1
duration: See below
area of effect: 1 mile radius/ caster level
casting time: 2d12 hours
Saving throw: See below
Magic users of higher level often summon familiars to assist them with various tasks. Indeed, a familiar can also be of considerable benefit to a lower level magic user (even increasing others’ estimation of his or her power), but the risks inherent in losing a familiar can be daunting to a weaker spell caster. To summon a familiar, the magic user must intone the words of the spell over a well-stocked fire source, sprinkling the flames with expensive incense and powders (100 gp in total value). The caster must maintain his or her casting for as long as necessary (2d12 hours) until a familiar arrives (or the casting time expires without success). The spell may be attempted only once per year, and the caster has no control over the type of animal that will respond. When it arrives, the familiar is a faithful servant and ally to the caster. Normal familiars have 1d3+1 hit points, are AC 7, and are as intelligent as a lower-than-average human. When the familiar is within 120 ft of the magic user, the magic user gains additional hit points equal to the familiar’s. However, if the familiar is ever killed, the magic user permanently loses twice the familiar’s hit points.
In spite of how much I appreciate just how flavorful it is for a spell caster to be accompanied by a familiar, the actual implementation from 1e-3.x was always a bit lacking, and usually dangerous. Taking it with you, anywhere, just turns it into a target, especially when facing an opponent who’s even vaguely intelligent. Yes, the bonus 1d3+1 HP for a first level wizard basically doubles your hit points, but the death of a familiar can also potentially kill a 3rd level wizard!
In 3.x the loss of the familiar hits a wizard or sorcerer as an XP loss (200/level). That's at least not potentially fatal...
4e handles familiars differently. The basic idea is that familiars aren’t actually real animals, they’re spirits or elementals, and when not actively being called upon by their wizard they basically pop off screen, and if they "die" you just re-summon them.
Now, this might seem to go against the grain of OSR grittiness, but honestly, that’s pretty much how I played it back when I ran 2e. I wasn’t going to penalize a wizard with a familiar so egregiously that they’d never take one.
So how do you run familiars?