Tettra - Tettra is a company wiki that helps teams manage and share organizational knowledge.
Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Perfect Wiki - Build and organize your knowledge base of company resources without switching between workspaces
Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.
Pages by Process Street - Pages is a free-for-everyone knowledge base from Process Street.Try it out!
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.