I agree with Talli's suggestion, "The Toolkit for Online Communities." It's short, strong, and focuses on what really sets OpenACS apart.
Would anybody _not_ say "enterprise quality" in their marketing? No way.
But would anybody not say "for Online Communities"? Sure. And that's what makes OpenACS unique. Why not use PHP/MySQL? Because it doesn't support online communities nearly as well.
I don't think you need the words "web application" in the tagline, because it is implied in "for Online Communities." How else are you going to support an online community but through a web application?
I also agree with the suggestion that to say "_THE_ toolkit..." grabs this "market space" very nicely. No one else can make quite that same claim, and AFAIK no one has. So now is a good time for the OpenACS community to grab it and run with it.
Shawn