I'm all for "Skills that make sense". My Intellegent Sword will not aid me in a Forgery, my Owl Familiar will not aid me in a Bluff. If the critter isn't moving, it's not likely aiding me in combat.
But how about Search, Spot, Listen? Could a Familiar aid in Spellcraft?
Now, here's the rub. What happens if, hypothetically, a PC has 12 Throwing Scarabs attached to his arms? (not so Hypothetically, I'm toying with a Rogue Master Thrower right now).
The Throwing Scarab itself has a +17 Hide, +5 Listen, +5 Spot. What happens if 12 Throwing scarabs Aid the host for a whopping +24 to a Listen check?
Provided those scarabs have an interest in aiding you, a willingness and ability to communicate to you all they hear, and no intervening sound barriers (like your clothing), I'd have no problem letting them aid another.
@
RUMBLETiGER I'm not sure why your sword couldn't aid you in craft checks - they're INT based. It might remember something from the time it was forged, and give you insight based on that (or any variety of other times it's been around a forge).
Some checks don't make sense -at all- for Aid Another. You can't exactly aid someone in Search, Spot, or Listen, although you can provide an additional check for them. They either see or hear something, or they don't. The same could be said for Autohypnosis, Control Shape, Concentration, Decipher Script, Knowledge, Psicraft/Spellcraft, Sense Motive, Speak Language, Swim, or Tumble. All things you either can or can't do, not a whole lot of "help" room.
I'd try to keep it at least a little logical. An intelligent sword could help you with Int, Wis, or Cha checks (within reason), an Ape could help with Str and Dex checks, etc, etc. As long as you keep it within the realm of expertise for the helper, I don't see a problem with it.
Forgery isn't about forging a weapon, it's about crafting a duplicate of official documentation, letters etc.
Also, aiding another doesn't necessarily entail granting a direct benefit to the one making the actual skill check. For example, if you set your familiar to "watch your back", while you keep a keen eye out in front, the familiar's aid another simply represents that you don't have to watch all angles of approach simultaneously. Since (if) the familiar has weaker senses than you, this only comes down to a +2 circumstance bonus on your own check, but it will help you be aware of approaching enemies.
If several people/creatures work together to make possible a positive result on whatever they're attempting *together*, the most logical mechanical representation for that is the aid another mechanic.
I could easily imagine a whole group of PCs scouting through the woods. The Ranger has the highest Spot/Listen, so
mechanically, he makes the checks while the other PCs aid another. In-game, the DM might still see fit to describe it the following way: "You're on alert and keep a constant lookout on all sides. Ranek [the Ranger] keeps warning one or another of you about likely angles of approach, posts one of you a bit to the back to keep watch over the rest of you, and takes point himself. [after rolling checks] Thanks to Ranek's guidance, Liovar [the Rogue] spots a glint of sunlight on metal just in time and manages to shout out a warning! Roll for initiative, everybody."
Granted, things like this don't work all the time, but if the situation and the used skillset is right, and the group is intent on working together and accepting of one guy taking the lead, it can work. Not for stuff like sneaking, but I'd totally do it this way for scouting.
EDIT: I just remembered an ad hoc rule I came up with a while ago, during an extended bout of in-game diplomacy: I allowed the party face to make the check, while the others were allowed to contribute (by thoughtfully stepping in once or twice, and by providing a suitably heroic-looking backdrop for the party's spokesperson). However, those failing their check to aid another would actually "hinder another" in this case. As a consequence, the half-orc Barbarian refrained from trying the check at all and just stood around silently, which was probably a good idea. The Cha 14 Conjurer, OTOH, felt like he could contribute at least a little, and since he was a man of outspoken opinions, this made very good sense in-game.