There are a TON of wikis out there - WikiMatrix lets you compare the tools. Keeping navigation design and usability in mind there are a few that call attention. Here are some of the ones we took a closer look at.

Google Pages

PB Wiki

Confluence

PM Wiki

Wet Paint

Media Wiki

BaseCamp Writeboard

Important Features:

1. starting the wiki.
2. adding pages, files, images.
3. editing and formatting.
4. saving and sharing.