37signals Once Product Announcement
world.hey.com
world.hey.com
Plenty of folks aren't willing to self-host services if they can help it, yet aren't satisfied with "Freemium" offerings (many of which tilt their funnel strongly toward the "-ium" end of the spectrum), nor are willing to pay "enterprise" per-seat rates in perpetuity.
Pay once, use forever (or at least as long as the company exists) with an open source escape hatch could be a successful (or at least sustainable) model.