N
Nobody
You're thinking of single-user PCs; Unix was designed for multi-user
systems.
You don't want one hung-over student to trash the library directory which
everyone uses.
It's like UAC in Vista, except Vista is torn between:
a) making software do it right (anything which a user modifies goes in the
user's "Documents and Settings" directory so it doesn't affect anyone
else), and
b) having to support the way it used to be done (programs using their
"Program Files" directory because they've already figured out where that
is and can't be bothered dealing with another directory).
So they compromised on trying to make case b) just annoying enough that
such programs would be at a disadvantage to their competition.
In Unix, the right way is the way it has always been done (assuming
that normal users can write anywhere outside of their home directory has
always been a losing proposition).
Programs normally locate data files using a sequence of directories,
including a per-user directory and a system-wide directory.
For a single-user system, you can make the relevant directories
group-writable for the appropriate group. This sometimes works for a
multi-user system where you can rely upon users not to shoot each other in
the foot, but you need to be careful about concurrency issues (i.e.
multiple users modifying a file or directory at the same time).
