Patch-Tag is a place to host darcs repositories online, for private project coordination, for sharing with the open source community, or even just for worry-free versioned backup of personal data.
Every darcs repository is also a darcsit wiki, if you want it to be. Gitit, an open source wiki tool, uses a version control back end (currently darcs or git) to store plain text pages, rather than a database. This means you can work on your wiki offline and push changes back when they are ready, if you like that workflow. Another plus: it's a lot easier to migrate a version control system-based wiki than a db-based wiki. Believe us, we've tried all kinds :)
Beyond the functionality already at hand, we are hard at work on collaboration features, tools for repo browsing and patch tracking, and other goodies.