PBwiki

• free
• read/write options for free accounts
  • public can read and public can edit
  • public can read but editing is password protected
  • reading and editing are password protected
• read/write options for paid accounts
  • everything offered for free accounts
  • password protection for individual pages as necessary

SeedWiki

• free

Wetpaint

• not yet out of beta, but looks very slick and promising

Wikispaces

• free
• read/write options for free accounts
  • public can read and public can edit
  • public can read but editing is password protected
• read/write options for paid accounts
  • everything offered for free accounts
  • reading and editing are password protected
• can import blog postings

XWiki

• free
• robust, with lots of features
• interface not very user-friendly

Wikipedia on Wiki Farms

• A comparison of Wiki-hosting services from Wikipedia