The first project I worked on at Google was a system for consumers to send money to each other. My team made a mobile app before I joined, but it "wasn't part of the product vision" for that to be in the Google Wallet app. (So I guess we'll flush hundreds of thousands of dollars worth of code down the toilet!) Our PM looked around for teams willing to use our service, and found the Gmail team very receptive. A few engineers integrated it in about a week and we launched that "send money" button on Gmail at Google I/O. Could never get over how Gmail had "send money" but Google Wallet didn't. (They eventually changed their mind, of course.)
I'll never forget the launch. I was sick, and sitting on the floor of a war room live-coding a script to give every attendee $1. Python, had tests, and it persisted its state to SQLite so that we could Control-C it if our service got too slow. It didn't get too slow; we did a lot of load testing and capacity problems and the launch was very smooth. Most fun I've ever had while sick. (10-year-older jrockway wouldn't go to work while sick, though.)