I use a varying number of spaces depending on the language I'm working with...php my spacing is very sporadic because my editor replaces all tabs with spaces automatically so it's hard to have some uniform spacing thing going on (as the auto-tab-indentation-selector-thing isn't very good, it'll randomly indent 2 spaces, or maybe 4, or maybe 7).
C++, however, I use 4 all the time, just seems like a decent in-between as I don't think 2 would make the code incredibly readable but 6 or 8 would make the code harder to read. In the end, I doubt I'll run out of hard drive space by using 4 spaces to indent as opposed to 2 or 1, heh, these days you can buy a gig of hard drive space for $1.
I start at left margin and add 2 per level of indentation.
I've never needed to go deeper than 6 that way.
I use tabs where each tab is 5 spaces. Never noticed the problem like drks posted example.
I use 4 spaces. In Code:Blocks a Tab is 4 spaces so I use that too. ;)