Generally, a wiki, usually (though not always) internal / accessible via VPN, though increasingly publicly hosted.<p>I've used, implemented, and/or administered: Twiki, Atlassian Confluence, Mediawiki (the Wikipedia engine), and others. My general preference is for tools which are already well known and supported, with Mediawiki scoring well on these grounds.<p><i>Simply accumulating documents does not constitute a knowledgebase.</i><p>The biggest problem is that information ages, it's hard to correlate with other information (so your KB becomes inconsistent over time), and you <i>need</i> to edit, prune, and review. <i>Failures of KBs to remain current indicate a problem of</i> process <i>not of</i> tools, <i>and must be remedied by changes to process, not tools.</i><p>I'm increasingly partial to systems which support creation of a book-type document, rather than merely an accumulation of articles (though there may be a place for both). In particular, a lightweight markup (my preferences are LaTeX or Markdown), a solid outline, and a regular review process, are probably a best-practice approach to this specific problem.<p>What I'd really like to see is a system which marries such an approach with conversations (effectively: a mailing list) and issue tracking (BTS or similar), and which allow for publishing a specific point-in-time document with revision control: PDF, HTML, ePub, etc. The latter are particularly useful as they allow for creating and distributing offline copies of documentation which can prove useful in circumstances in which online access isn't available or convenient (e.g., the server / colo / network has melted down).