Pavilion. A site that focus on raising awareness of healthy multiplicity.
Astraea's Web. A great site with lots of information on multiplicity, with a good deal of focus on how MPD/DID is different and the mental health side of things. Most likely the first website devoted to multiplicity.
Collective Phenomenon. Another awesome site with an FAQ, a page dedicated to busting myths and misconceptions and articles written by various system members.
Article on DID. An article that explains why DID (and the MPD) model doesn't work for all people, and how the mental health understanding of multiplicity is severely lacking.
Amongst Ourselves, by Tracy Alderman and Karen Marshall. Karen has a multiple system of her own, and while I severely dislike the section on "Satanic Ritual Abuse" (which was proven to be a fabrication after many years of research by police), the rest of the book is useful and uplifting. Karen and co come from the DID model of multiplicity, so their system was apparently created from abuse, but the writers do point out that not all DID comes from abuse, and some wording leaves the reader to assume that "DID" can mean anything that a system thinks it means (self-determination).
A short history of soulbonding. Blunt and to the point, this article is a good read and a great reminder that what works for one person doesn't work for everyone.
Soulbonding(dot)net. A site with plenty of articles and personal experiences.
Soul Whispers. A decent site with a basic explanation of soulbonding.
Otherkin(dot)net. One of the largest websites about Otherkin topics there is... lots of articles and information.
Otherkin Alliance. A website with articles, an FAQ and forums.
Otherkin FAQ. An extensive FAQ covering Otherkin topics. This FAQ also addresses feelings of doubt and coping with memories/feelings, which are important and often glossed-over topics.
From Fiction. The owner of this website uses the term otakin/otakukin, which was a term initially created as a joke and has since become a bit derogatory, but a horse is a horse is a horse. It's the same phenomena, and there is some decent information to be gleaned from this site. It's not easy to find mediakin websites in general.
Soulbonding(dot)net article. While it's a short article, it gives one the basic idea of fictionkin/mediakin.
Kinjou Ten's article. The person who coined the term "otakukin". It's an interesting read.
String Theory website. Because quite a few people like to use String Theory and the idea of a multiverse as an explanation for their experiences, here's an easy to understand website about the theory.
Closer To Truth: Multiple Universes. A BBC TV show, Closer To Truth talks about a variety of topics with scientists, authors and the like about their perspectives. They are free to watch. This one is about multiple universes.
From Universe To Multiverse. A YouTube video clip from Dr Michio (theoretical physicist) of an explanation of the multiverse theory.
Welcome To The Multiverse. A Discovery Magazine article.
The Case For Parallel Universes. A Scientific American article.
Thébaïde. A good website about therianthropy, created by a therian.
Beyond Awakening. While technically an otherkin site, it includes therianthropy here and there.
WikiFur article. The article for therianthropy on WikiFur.
Therianthropes. A website with a hefty dose of information.
Moonscape's article on non-spiritual therianthropy. For the less spiritually inclined.
Project Shift. A good website with many articles.