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One of the alumni companies was PagerDuty, who went on to get YC funding in the last (YC S10) class.

Rapportive (YCS10) were part of the 'accidental incubatuor' that kinda got the ball rolling. (http://webcache.googleusercontent.com/search?q=cache:T3Vxyoj...)

Here's Neil's original article on the Business of Software blog about the 'accidental incubator':