The most productive week of my life, every other employee of the small company I was working for took off for a trade show. Most of our customers also happened to be going to the show, so the support email was pretty dead. There was also a blizzard that dropped about four feet of snow that week, so the entire office park, and for the most part the roads coming and going were a ghost town.
I got up in the morning, had breakfast, loaded up my dog and drove in. My dog bounded around through the drifts off-leash for a while, then we went inside and got to work. I discovered how to do things that we didn't think were possible, abused APIs in ways that shouldn't have worked, but did, and bent the bits to my will. It opened up a whole market that we had been previously forced to concede. I smashed together a workable implementation of one of our products for a new platform that we used, essentially unchanged, for a year and a half until the platform finally released a real API. By Monday when everyone came back, it was a fait accompli, and I demo'd it. I didn't speak to or hear a word from another human being the entire week. It was glorious.