I've been quite happy with Atlassian Confluence. There's a rich set of plugins available, the formatting is fairly complete and reliable, and they have both cloud-hosted and conventional licensing options. Some of the features we find especially helpful:
* Page templates
* Revision history with visual diffs
* Flexible Notifications
* Page hierarchy, with navigation pane
* Code blocks with syntax highlighting
* WYSIWYG editing, with auto formatting: start a paragraph with *, press the spacebar, and you've begun a bulleted list, like Google Docs. Enclose in double curly braces for fixed-space code formatting.