Only one I topic today - since today is, in fact, technically tomorrow (late night gaming, sorry, folks) - I might go back and fill in a couple of extras as I think of them.
My thoughts on magical invisibility are largely informed by a Dragon Magazine article - Seeing is Believing, by Geoffrey Meissner (Dragon #105, January 1986, available in the Dragon Magazine CD archive - my players can read this article if they like, since I have a copy of the archive).
Invisibility basically breaks down into three types - Mental (invisibility invoked by the caster in the target's mind), Illusionary (invisibility invoked through the use of illusion and misdirection), and Physical (illusion invoked by the physical bending of light around the subject in some fashion). Nearly all known forms of invisibility fall into the first two categories - only a very small handful of magical devices, and no known spells or conjurations, allow for physical invisibility.
Mental invisibility can be quite effective - but is limited to targets that have a working mind of some kind. Spells that grant invisibility of this type are usually of the Enchantment or Charm schools of magic, in whole or in part. These spells are somewhat more rare than those which grant illusionary invisibility (below).
Illusionary invisibility is much more common than the other two types, and is extremely potent - but limited in scope and aspect. Unlike mental invisibility (which often also covers other senses - it acts more as a sensory edit that wipes all evidence of the target from the victim's mind), illusionary invisibility is often limited to a single sense (sight), although Improved Invisibility at least covers some level of deception with regards to other senses, and more puissant spells can act in many ways like mental invisibility does.
Physical invisibility is by far the most potent kind, for it works completely in the "real world" - but it is also the most difficult to manifest, and the most rare form of invisibility. Like Illusionary invisibility, it only covers sight - including infravision and ultravision, among others - but unlike those arts it cannot be extended to cover other senses. Unlike other forms of invisibility, until the duration of the effect wears off (or some other action is taken to nullify the effect), it does not degrade or fail to function - which can be both a blessing and a curse.
Invisibility is certainly a known spell in most corners of Malaster, and the general assumption is going to be - when faced with actions that are likely the result of an invisible subject - that this is, in fact, the result of an invisible prankster or assassin, rather than those of demons, delusions or madness, etc. This does not, however, mean that invisibility is worthless - quite the contrary, for defenses agains the myriad forms of invisibility are hard to arrange, expensive, and require near-constant upkeep, in most cases - but it does mean that invisibility is far from unknown, and it is an effect taken into account when dealing with strange crimes, etc. and when arranging for security, and the like.