For me, it's Node.JS. Most useful new tool since jQuery. I wrote a domain name generator with it a couple months ago:
It handled simultaneous Reddit and HN front page traffic without any problem. I'm pretty sure server load never passed 10%. The script ran for more than a month straight and performed 120k+ searches without crashing.
Considering the first Node.JS release was only 18 months old, it's amazingly stable and scalable.