Hey just wanted to give some context - I'm from the Teespring team, we signed on a couple months ago. Browserling has been incredible for us in terms of diagnosing fringe bugs and testing new features across multiple browsers quickly (especially javascript/dynamic elements).
I should clarify that I posted this, not Browserling. I'm just a happy customer.
I put YC Reject in the title because I thought it would be interesting to explore the companies who didn't make it into YC, but have still grown into successes.
Looking forward to seeing what's next