Years ago, working on GNU documentation, RMS instructed me to use two spaces. The documents were processed into HTML, TeX, Texinfo, and plain text, the former two of which auto-collapsed two spaces into one, and the latter two of which were often viewed with a monospace font, thus benefiting from the extra space.

I carried this practice over into the rest of my writing, although recently I have noticed that some web systems do not auto-collapse two spaces, and when filling out forms online I've generally reverted back to using single spaces to ensure proper spacing.