two spaces for all informal communication. it's important for me because i don't capitalize; i need the extra space for readability.
anything that's going to be printed, i use one space. if it's sorta formal, but not intended for print, like an emailed letter or something, then i usually use two spaces. because that's my preference and my default. but i'll sometimes use one, if i feel i'll be judged adversely for using two.
great thread topic, btw.