I've lost count of the number of cool technologies and tweaks we've built as a result of 5Ys. For example, occasionally one of our engineers will decide to build his own website, and use simpletest to unit test his PHP because that's what we have at work.
That engineer invariably complains about how bad out-of-the-box simpletest is compared to the house model we've cooked up. I've heard "It doesn't even have X!" many, many times. More than half the assertions in our php testing framework are custom-made, all the web gets and posts assert on the response code automagically, etc. And that's not even counting how much support we've ended up building into it for our other technologies - automatic detection for failures in memcache, SQL, and what-have-you. Or the other testing frameworks we've rigged to run within it, like YUI test and selenium. All because of 5Ys, and that's a minority of the good 5Ys have done for us.