The indentation of source code is always a controversially discussed topic. Some prefer spaces, while others like tabs. And those who have no clue, don't even care about such ridiculous things like code formatting at all.
"Tabs are 8 characters, and thus indentations are also 8 characters. There are heretic movements that try to make indentations 4 (or even 2!) characters deep, and that is akin to trying to define the value of PI to be 3. [...]
Now, some people will claim that having 8-character indentations makes the code move too far to the right, and makes it hard to read [...]. The answer to that is that if you need more than 3 levels of indentation, you're screwed anyway, and should fix your program.
In short, 8-char indents make things easier to read, and have the added benefit of warning you when you're nesting your functions too deep. Heed that warning."
So with all that said, below are some numbers and charts for a small selection of highly successful open source projects. The source code of these projects was inspected line by line. Each line was then counted into one of the following categories: NOT, TABS, SPACES, BOTH or BLOCK_COMMENT. And for lines with trailing spaces, the TRAILING category was updated additionally. The BLOCK_COMMENT category is a special one and only used for languages which support block comments. A line belongs to the BLOCK_COMMENT category if it begins with a block comment or is part of one. The used block comment detection is not perfect, but good enough for most cases.
P.S. Personally, I like tabs.