Wiki.com is currently featuring CentralDesktop as our wikiware partner for those who prefer a hosted version. (Best of the Web 2006, Business Week.)

Their free version includes 2 hosted wikispaces of up to 5 members each, 25 MB storage, WYSIWYG editing, and page history.

It is fully scalable for enterprise, but fees for extra users or more storage space apply. Audio conferencing, IM support, and other features are also available.

Features include:
Project Tracking
Searchable Discussion
Shared Calendars
Word, Excel, Powerpoint, PDF
256-Bit SSL Encryption

Ready to start your wiki? 
What exactly is a wiki, and what can it do for me?  
Three questions to ask before you choose a wiki engine