Jeans and t-shirt.
I've worked at one tech company in a department that was particularly desirable, doing crazy R&D projects, and we were required to dress more smartly. The CTO was also apparently quite unhappy about people dressing down more widely in the dept. I was once lightly reprimanded for attending a standup without shoes on.
I can see some reasoning in it, but it was never an official part of the dress code. As a result it felt awkward, elitist and made the whole company feel a tad fusty and out-moded. It ultimately formed a small part of my reasoning to leave.
Counterpoint: I still dress up to speak/present to the C-team. It just seems like a good idea. It's a bit contradictory given my previous points, but to an extent it puts me in the right frame of mind. And at appropriate times I also follow the Dress For The Job You Want ethos.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.