Docs, whiteboards, and tables in one tree, collaborative in real time and connected to the tasks they describe. It won't replace your whole wiki, but it keeps your design where the work already lives.
Free while in beta.

A rich-text editor for specs, systems, and internal notes, with images, tables, mentions, and live cursors. Documents can nest their own child pages, so a system and its details stay together.

Folders, documents, whiteboards, and tables live in a single explorer. Drag things around to reorganize, and nest a bestiary under combat under gameplay, the way you'd map it in your head.

Throw ideas onto a shared canvas before any of them earns a spec. Sticky notes, shapes, and everyone's cursor in the same place.

Tables with typed columns: text, number, select, tags, date, checkbox, URL, or a project member. Colour your options, sort and search, and freeze the first column. Handy for asset lists, quest logs, and bug trackers, or anything you'd otherwise force into a spreadsheet.
Every document, board, and table keeps a revision history you can restore from. Turn documents, whiteboards, or tables on or off per project. Folders are always there.
Free while in beta.