We've had the chance to work with so many awesome teams like Pebble, Timex, Boosted Boards, and a great group of the biggest projects from Kickstarter. It's hard to list them all but here's a sampling: https://www.trycelery.com/customers

As for custom and DIY checkouts, August.com is one of our coolest examples of a checkout built on our Celery API and there are some exciting ones in the works right now. Stay tuned!