[3.5] Lists of Necessary Magic Items

[Sception]

Not an item, so probably doesn't belong on the list, but worth mentioning the Necropolitan template in Libris Mortis, with rules and costs for any character who wants to add it mid game (1k gold, about a level's worth of xp, min level 3). Grants the undead type with all the usual benefits and penalties, including immunities to many of the effects this list concerns itself with, as well as the usual drawbacks (d12 hit die + no con mod is a neutral or positive trade for most arcane types, and even some diviners or skill monkeys, but bruisers are likely taking a hit to their HP; plus vulnerability to turning/rebuking, death at 0 hp, opposite effects from heal & inflict spells, etc).

It can be a good choice for characters looking to snag immunity to a pile of these effects independent of class features or feats and at a cost far below that of most of these items. Loss of xp is a pain, the whole party doesn't want to go necro all at the same time, but you will earn it back in a couple levels.

Since it has a GP cost and is independent of other race/class/feat concerns, many players might consider it alongside items in their heads.