Depends, really. If I'm typing about Dunkin' Donut's, I spell it donut, (which isn't underlined in Firefox using the Canadian English thing), though if I'm typing about other stores- like Tim Horton's or IGA/Inter Marchet, I spell it doughnut, (which doesn't get the red underline either).

Doughnut. Whenever I type donut, I feel lazy ):

Donut, because that's the only correct spelling in Dutch ..I feel offended now, I'm not lazy D:

Haha it depends for me..

Sometimes I type/write it both ways.. :)

doughnut

Unless it was in context I don't think I'd even realise what a donut is.

Doughnut. I was taught to spell it that way. xD

I alternate, I guess. If I want to look professional, I use 'doughnut'. If I don't care, I use 'donut'. I learned it 'donut', though - used to go to Dunkin Donuts alll the time with I was little. ^_^