Eidolon (Dungeons & Dragons)
Eidolon and its variant Eidoloncer are a class and half from Ghostwalk. The only real rival it has for the title of most obscure 3E base class made by Wizards of the Coast is the OA version of Shaman. In the Eidolon’s favor for that title is that while Shaman doesn’t have any abilities a core class doesn’t and has nothing that really stops it from being used in any world, Eidolon requires being a Ghostwalk-style Ghost (which makes you an Outsider, not Undead) with all baggage that carries. In the Shaman’s favor is that, despite Eidolon being a 20 level class that "follows the same format as those found in the Player’s Handbook", it can’t be taken at level 1, raising the question on if it truly counts as a base class. Of the 20 levels, you'll only ever see a max of 19 by default since you need to have a hit die from elsewhere (typically a base class, but Racial Paragon Classes and racial hit die also work).
As for the class itself, in addition to the abilities being a ghost gives you... a bunch of bonus feats, that have to be [ghost] feats, on a weak chassis of full BAB, d8 HD, 2 skill points and a high will save. While on first impressions this may sound a lot like the notoriously crappy Fighter, there is the major difference in that, unlike [Fighter] feats, [Ghost] feats are actually good. In-fact, the way these feats work, they resemble the talents of d20 Modern and (in the sense that they're actually worthwhile and a functional class) the later Star Wars: The Roleplaying Game: Saga Edition but with the ability to sacrifice normally acquired feats to get more. These feats come in six “paths”, Corruptor (debuff/damage), Dominator (take a wild guess), Haunt (fear), Poltergeist (ghost touch, unfortunately mostly useless since Dominator and Shaper can do what it wants to do but better), Shaper (make shit from ectoplasm), and Traveler (movement).
Eidoloncer is basicly the same, but in exchange for less feats, lower BAB/HD, and a change to class skills, they advance spellcasting at every level (the table doesn’t show it at level 20, but text does and text trumps table and you can’t take it outside of Epic anyways). Since most 3E casters have few if any class features past level 1, this is a pretty good option and is a strict upgrade to Sorcerer in every way if you don't find an obscure feat that requires a high Sorcerer level (instead of the more common requirements of a high caster level or being able to cast X level spells). A Wizard or Archivist Eidoloncer may have difficulty keeping their spellbook around, though some alternate class features can make that less of a big deal for a Wizard.
Eidolon are obscure enough their tier is rarely discussed, and their highly variable build options further complicates the question. Just being incorporeal alone is a huge boon, especially once the Book of Bad Latin included the feat Ghostly Grasp that lets you ignore most of the negatives of being incorporeal. Normal Eidolon is likely a Tier 3, or at least a high Tier 4. Eidoloncer advances casting at every level, and will maintain the tier of its original class (tier 1 or tier 2) unless you do something dumb like advancing a casting class with useful class features or advancing a partial caster.
Pathfinder[edit]
You're likely looking for the Summoner, which has a walking class feature of that name.