Confluence by Atlassian has worked out great for me in the past. You can track changes, control rights to edit (and more) and take advantage of lots of features. It also integrates with other Atlassian tools. Let me know if you'd like further info or details on my past implementations.
Knowledge management is a broad category. It would help to know what you intend to use it for.
If you are a service company, for example, a product like ZenDesk or JIRA HelpDesk will contain the ability to log categorized answers, provide search features, etc.
For general wiki documents and searchable content, we use Confluence from Atlassian at work. If you don't know what you want or need yet, try one of the inexpensive products like Zoho Wiki in order to learn. After using that for a month or two, you'll be in a better position to pick out the right product for long-term use.