Tacking on a few more recent additions to the previous write-up: scribd, Citizendium, PlanetMath, ScienceZine, Refdesk, Encyclopedia Mythica, dKosopedia, wikinfo, The Encyclopdia of Diderot and d'Alembert, Quora, and fictionaut.
Each of these sites differs slightly from the other and all embody a part of what E2 encompasses.
Mouq described scribd pretty well already, though prior to reading his writeup, I knew it only as a place to find pdf files of journal articles or historical documents—not original material.
Citizendium is about five years old but I've never heard of it. It seems to be very similar to Wikipedia, albeit replacing the anonymity with a higher standard for content. Maybe in time it will surpass granddaddy wiki. More likely it will exist separate but more pure, not unlike the distinction between conventional and organic produce.
PlanetMath began around the time that the first writeup in this node began and seems fairly comprehensive. If I was in taking math classes now (or had I known of this while in high school) I would definitely use this as a resource. The site bills itself as "Math for the people, by the people."
The cool thing about this encyclopedia is that you can browse it the same way you would a book. There's a list of articles by subject—which ranges from General to history and biography to theory to functions to calc to stats to computer science to physics of every flavor to social math to biology to education.
Articles within each of those sections are separable by type: collaborations, exercises, research, education, reference, and recreation. You can also view the entire site alphabetically, by popularity, last modified or last added.
There's not much there aside from math. I guess you have to know where to look if you're looking for jokes on that site.
ScienceZine began in 1998 and the layout looks a little dated. It has some of the same topics as PlanetMath does, along with a section on the philosphy and history science. (I'm sure that their biology sections are more taxonimcal and less analytical.) The cute thing about this site is that it is a true labor of love: conceived by Alan and Lucy Richmond, who met in 1988 while working on data for the HST. According to their bio, their site was the first to be written in xhtml! Some of their material seems to be repackaged wikistuff. In addition to an online encyclopedia, they also made some babies. (which probably explains why their site is not so robust—for example, I clicked on their link for HST and was taken to a page full of stuff where I had to hunt to find which HST they meant.)
The Richmonds also operate an online science shop called Curious Minds that would be a good place to shop for little budding scientists. ScienceZine is also subsidized by some flashy ads (no pop-ups, thankfully).
Refdesk is a portal that wants to be your browser's homepage. It provides ready access to all kinds of information and search engines that could be obtained rather easily by a good google. Refdesk is a "fact-checker" and not an online database that allows users to create new content. Even so, I am including it in this list as it does do part of what e2 does. If you wanted to know what the sky looks like now or what today's earthquake/literary/music/late night joke histories are, then go there. It has all sorts of handy links for all sorts of interests.
Encyclopedia Mythica, as the name suggests, deals in myths from different cultures. It's a curiosity for historians and anthropologists. Unfortunately the site seems to be abandoned and most of the articles are brief and lack links. The site began 17 years ago(!) and, as of August of 2009 was gearing up for a big reboot at the start of 2010.
It's ironic how a site run by peope who deal in the remnants of ruins to itself be in such a stasis. Maybe if you poke them you could get your name added to their list of contributors by submitting something.
dKosopedia is a "collaborative project of the DailyKos community to build a political encyclopedia". The site is made with media wiki technology, which means it looks like Wikipedia. Anyone can contribute. This site began in 2004 and has an unabashedly liberal/leftist take on how the USA relates to everything. Not very active.
Wikinfo is probably the most recent entry in this listing. It is a multi-lingual wiki project which "provides a platform for the meshing of encyclopedic material, original and creative work and public domain material to further education and information." Far as I can tell, it has only been in existence for a few months now, so there's not much there—as evidenced by the red links which indicate empty pages.
To further quote from their nascent self-description: "Original Poetry may be posted in the Poets corner and original fiction may be posted in the Writers corner. Wikinfo differs in editorial policy from Wikipedia. Generally, the main article on a subject should present the subject in a positive light or as a concept which makes sense. Alternative or critical perspectives should be placed in linked articles, Wikinfo welcomes editors from Wikipedia and offers much more opportunity to edit freely. Original research and original ideas are welcome."
So I suppose that Wikinfo is more of an alternative to Wikipedia than E2. Evenso, the site does have definite potential.
Le Encyclopédie de Diderot et d'Alembert is a collaborative project hosted by the University of Michigan. The ongoing task is to translate the world's first encyclopdia from French to English. They are actively seeking volunteers. Individual articles are not linked but the entire thing may be viewed by topic or title, in either language. The original illustrations are also reproduced.
Visually this site has a nice clean, uncluttered layout. The type is on the small side though. This site is worth visiting if only for the included Auxiliary Materials—some nifty outlines and diagram mapping out Knowledge and history and such.
Quora is a site which claims to have a high number of pedigreed contributors. Unfortunately one must join in order to learn much there.
Fictionaut was founded by Carson Baker and Jürgen Fauth in May of 2008. Despite being an invite-only deal, this site is very active. Fictionaut describes itself in a way that is very similar to E2:
Fictionaut brings the social web to literary fiction, connecting readers and writers through a community network that doubles as self-selecting magazine highlighting the most exciting short stories, poetry, flash fiction, and novel excerpts. Judging from the site's blog at least a few people there have graduated to print.
Each piece published has a sidebar for responses by users—the sort of chit-chat which often occurs as a private message here at e2. Note that these comments are viewable to those not logged-in.
One blog review I read about Fictionaut called it a "Flickr for writers" but from poking around on the site, it seems more like Facebook to me. Many people on the site have a little icon with their face and use their real name.
Which reminds me of one of the first private messages I got after joining e2 and being told of the security gained by the anonymity provided here:
It is all about the text.
I'm still inclined to agree with that sentiment.
Addendum: Buried in the mess of softlinks for this node was ek$i sözlük. It is a real site and probably a lot of fun if you are Turkish.
One of our most epic storytellers here at E2 tipped me to AbsoluteWrite, describing it to me as "a writer's site dedicated to the various ins and outs of (compared to E2) serious writing".
As someone who considers all writing to be serious, my curiosity was piqued. It's a writer's site, all right—a hub in which bloggers interact on brisk forums, vying for the attentions of literary agents. No actual writing to be had there, but links outside abound—as do motivational pap and adverts for classes, all akin to Writer's Digest.
If one seeks Alternatives to Everything2 in the aim of becoming a better writer, I'm convinced that dead trees by dead people are the way to go.
ink node is a (curated?) online literary thing-y where lotsa folks seem to be using their Real Names and listing where they went to school to learn How To Write. There's a poem there called Saran Wrap Orgy that doesn't mention Roy Orbisson. None of the "nodes" are linked... meh
also found something called "helium" which seems like about.com crossed with facebook. supposedly publishers troll that site. After poking around there for a few minutes, I was deterred by the heavy ad-content. Not only are there adverts on every page, but the site randomly assigns advertising to words willy-nilly. For example, the word "add" linked to ADHD meds. And the quality of writing was pretty abysmal.