Wikispaces - A free wiki host providing community wiki spaces, visual page editing, and discussion areas. - http://www.wikispaces.com/
Wik.is - Free wiki hosting service based on DekiWiki. Instant registration, URL format "http://yourtopic.wik.is/". GUI editing, login required to edit. Formerly wiki.com. - http://wik.is/
Wikidot.com - Free ad-supported wiki hosting service for non-commercial use, based on Open Source wiki software. URL format is "http://yourtopic.wikidot.com/". Wiki syntax only, interactions with other services, extended and extensible wiki syntax, uploads, f - http://www.wikidot.com
On-wiki - Free wiki hosting based on MoinMoin Open Source wiki engine. URLs of form "http://yourtopic.on-wiki.net/". WYSIWYG editing, file upload, custom permissions, ads on each wiki page. - http://on-wiki.net/
XWiki.com - Free wiki hosting site with text ads, using Open Source XWiki software. URLs in format "http://yourtopic.xwiki.com/". Immediate setup, WYSIWYG editor, photo album, calendar, wikiblog interface. - http://www.xwiki.com/
Peanut Butter Wiki - Free ad-supported wiki hosting with paid upgrades available. URL in format "http://yourtopic.pbwiki.com/". Requires shared password to edit, no GUI, file upload, API. - http://pbwiki.com/
JotSpot - Free wiki hosting service; custom software includes calendars, photo galleries and spreadsheets. URL format "http://yourtopic.jot.com/". GUI editing, themes, access control. - http://www.jot.com/
Wetpaint - Free wiki hosting service with proprietary software, GUI editing interface, and instant site registration. URL for wikis is "http://yourtopic.wetpaint.com/". - http://www.wetpaint.com/
Wikia - A collection of freely-hosted ad-supported wiki communities using the Open Source MediaWiki software. URLs like "http://yourtopic.wikia.com/". Requires 2- to 10-day approval process, single sign-on across all Wikia sites, file uploads, transclus - http://www.wikia.com
Schtuff - A free wiki farm with flexible membership and permission options and file upload. Up to 200 MBytes and 3 wikis allowed per user. - http://www.schtuff.com
EditMe - Provides wiki and content-managed website hosting. - http://www.editme.com/
Swiki.net - A wiki farm offering free hosting of Swiki (Squeak Wiki) based wikis. - http://www.swiki.net/
SeedWiki - A Wiki Farm where basic Wikis are free. Offers plugins that allow applications and web sites to be built on a Wiki Framework. - http://www.seedwiki.com