Hum.., How about a Free, beta web app from Heeduser. heeduser.com I vote for it. Since it's - (1) Full fledged community with comprehensive collaboration platform. (2) Modern usability with social integration. (3) It's simple, clear, beautiful and free.
Really a great source for any one who wish to opt for any of these items.. How about adding a new beta program Heeduser - heeduser.com (Community powered customer and visitor engagement app)