Tabs v spaces
As Peter said, the most important part is consistency. Mixing tabs and spaces together makes no one happy.
I will throw out a couple arguments for tabs here:
Indentation of code, markup language, et. cetera is meant to show the structure of whatever. Tabs are a logical, structural element. Spaces are physical.
Different users may want different sizes of indentation. If i want 4 character indentation and my coworker wants 2, tabs can be set to show our preferred width.
Seems for the most part that spaces are winning out though.