Hackpad includes easy to use privacy controls: You can set a page to be public, open only to people who have its link (like the mode Google offers for sharing Picasa albums), or open only to people you've explicitly invited.
Hackpad does not yet offer commercial plans, but its free offering is full-featured, and feels amazingly fun and simple to use. If you've ever felt frustrated and limited by your current collaboration tools, take Hackpad for a ten-minute spin. Careful: It's addictive.
Note: The Download button takes you to the vendor's site, where you can use this Web-based software.