Comparisons of wikifarms can be found here on Wikipedia, and here at editthis.info, and below.

@Wiki(Atwiki)

Includes a WYSIWYG (What You See Is What You Get) and Wiki-mode editor. Users receive a sub-domain and personal wiki upon signup. Unlimited pages and users.English documentation is minimal and it is unclear how to backup pages. Now allows MediaWiki-like monobook.

ClearWiki

Extremely easy to use. Includes WYSIWYG editing, raw HTML editing, blogs, RSS feeds, to-do lists, meta-tagging, bookmarks, comments/discussions, member directory, access control lists, document management, full-text search inside content and common files (eg MSOffice), version history of content and files. Free wikis, advert supported wikis, and subscription wikis available.

Cospire

Includes a WYSIWYG (What You See Is What You Get) editor, category tree, annotations, article ratings, member profiles/reputations, and options on article and member permissions. Although current advertising is limited, ads might be added sidebars to cover hosting expenses.

Deki wik.is

Designed to balance ease-of-use with rich features, Deki Wiki allows you to create, organize, aggregate, and mashup information. With our WYSIWYG editor and XHTML diffing, Deki Wiki eliminates the need for complex knowledge of technical wiki markup in order to contribute content. Under the hood, you'll find a wide variety of extensions, the mashup framework for embedding content from diverse web services including Google, Yahoo!, and Microsoft Live.

Frihost

Free webhosting that supports most wikis. Manual installation of a wiki is required. No ads and users receive their own subdomain. Users need to be active at the forum to keep their wiki.

GForge

People who put their source code at GForge also get a free associated wiki.

infogami.com

Hosting based on a custom Python-based engine. Fast setup. No banner ads. New wiki receive their own subdomain. Features: customizable templates; Fine-grained permissions; activity graphs; built-in weblog.

Intodit

User friendly WYSIWYG Editing, Custom Page Hierarchies, Comment and Reply Forums, Page Locking, Keyword Search, Skins, RSS, Profiles, Site Activity Report, Widget Support e.g. YouTube and Google Video.

MemeBot

Hosting based on Oddmuse. FTP and CGI access allows expert users to customize their configuration. Users receive their own sub-domain.

Netcipia

Free public and private wikis, with blog feature. 2GB per wiki created, no time limit, no # users limit, myNetcipia to manage the list of wikis created. Right management, wysiwyg editor, spaces, RSS, comments, attachments, code. Not showing Unicode characters properly in the source yet due to the limitation of XWiki. Non-Western users may have to wait for this issue to be solved.

OddWiki

Hosts wikis up to a certain size with no advertising. Based on the MotherWiki concept. Simple support for backup and migration to your own Oddmuse wiki.

PeanutButterWiki (PBwiki)

Public or private ad-supported wikis. Offers quick setup, SSL logins, RSS 2.0 & Atom 1.0 feeds, .ZIP backups, subdomains, 3 free "skins", and 10 Mb of file upload space. No limits on number of pages, number of wikis, or page size. Paid wikis remove ads, get 1000MB-5000MB of storage, offer full-encrypted sessions, and CSS customization. Wikis use a simple shared edit password to prevent spam; paid users can set different passwords for reading, editing, deleting, and admin.

ScribbleWiki

ScribbleWiki is now down, permanently.

SeedWiki

Has hosted free and paid wikis since 2001 using open source software. Supports WYSIWYG (What You See Is What You Get) editing, bare-bone editing, backup capabilities, subdomains, and CSS (Cascading Style Sheet) support. Some of the locking/membership/DNS features are only available for paid accounts. The code base can only run on a proprietary Bluedragon CFML J2EE server or a proprietary Coldfusion MX J2EE server (although it's currently being ported to some other more open platforms).

ServerSideWiki

Collect notes, todos, personal information, you name it. Based on TiddlyWiki. Hosts free and paid wikis.

SourceForge

People who put their source code at SourceForge also get a free associated wiki.

TiddlySpot

Public or private access to wikis based on TiddlyWiki yet fully compatible with MonkeyPirateTW, MonkeyGTD and d3 plugins. Brilliant for notes, todos, follow-ups. Easy for download, edit off-line and keep the online version up-to-date, or even easier to edit on-line (password protected). Hosts free wikis.

WetPaint

True WYSIWYG Editing, Custom Page Hierarchies, Comment and Reply Forums with Comment Ratings, Access Control, Keyword Search, Tags, Tag Filtering, Skins, RSS, Page Locking, Profiles, Site Activity Report, Page and Comment Watching, Uploading YouTube Videos.

Wikia

Community support and development through advertising-supported wiki hosting. Uses the MediaWiki engine with WYSIWYG editing, easy image and video insertion, and a number of extensions. Wikis are community owned, not owned by the person "founding" the wiki. All content is freely licensed under the GFDL to make it compatible with the Wikimedia projects. See Start a new Wikia to create a wiki instantly. Offers free database backups, full CSS customization, spam blacklisting and vandal patrol, help from Community Team.

Wikidot

Offers free Wiki sites in the wikidot.com subdomain and 1 free custom domain mapping. Uses the modified Text_Wiki engine to produce valid XHTML. Growing number of extensions. Several available licenses (Creative Commons, GFDL and others), customizable themes, AJAX interface, no size/traffic limits, full-text search, RSS import and export (for page changes, forum discussions, favorite items, account notifications etc.) and many more features.

Wikihost

Hosting using the GeboGebo engine with all its features (as AutoLink, RSS-Feeds, file and image uploads, etc...). Supported by Google text ads. Offers a scriptable export function for backups.

WikiLot

Wikilot closed down in August 2008. Afterwards, domain and backup data have been purchased by Wikia to offer a new home to the wikis that were hosted there.

Wiki-site.com

MediaWiki with WYSIWYG farm with unlimited pages and users, wiki spam protection user rights control. Interface in 150 languages. Fast setup. A free *.wiki-site.com domain is offered. Google text-ads added to the right sidebar.

Wikispaces

Public (free), protected (paid for), and private (paid for) wikis. Includes WYSIWYG and plain wikitext editing and a personalized sub-domain. Supported by text ads; no limitations on number of pages, members, or space usage. Includes integrated discussion areas and RSS feeds for page changes and messages. Backups of current content available through the site in zip or tgz form. Backups of historical information available by request. Run by Tanigent LLC using proprietary wiki software.

Wiki Spot

Public wikis (free) intended for a community, be it a city, common interest, field of study, or project with several participants. (No private or commercial wikis, please.) Hosted by a non-profit organization supported by donations, you can host your wiki in an established wiki community that can help you out with your project. No advertising, user accounts work on all wikis, use bookmarks and track changes across multiple wikis. Customizable CSS by wiki and user CSS. Simple, intuitive syntax. No limits on pages/disk space. RSS feeds available on all pages. Easy to create a wiki and configure settings. Run on Sycamore, creative commons licensing.

Wikkii

Free wiki hosting allowing full back-end control with FTP and Cpanel access. Wiki farm service also available.

XWiki

Hosting based on the XWiki engine. Features include quick group editing, version control of documents, attached files, searching, wiki syntax, and rights management). Pages can be exported to PDF. Full MySQL exports can be provided by the host admin and installed using the open source XWiki engine.