“Why isn’t Arial in my LaTeX installation by default?”

“Or Cooper Black, or a bunch of other fonts? They’re free, aren’t they? They’re already there on my computer!”
If you’ve ever wondered about this, this article on Free Fonts: Free Is Not Always Free may help you understand some of the underlying issues about font licensing. (Some people would argue that the M$-bundled fonts are of inferior quality compared to the real stuff anyway e.g. Times Roman, Helvetica etc)
On the other hand, rest assured that all is not lost as there are free clones of many of such fonts that you can use in your LaTeX documents anyway.
For example, \usepackage{mathptmx} loads Nimbus Roman, a look-alike for Times Roman, while \usepackage{helvet} loads Nimbus Sans to stand in for Helvetica, which is pretty much indistinguishable from Arial — to the untrained eye anyway.
One last thing: You can still use some non-free fonts: install them using the getnonfreefonts script.
One really last thing: The last thing I want to do is to start a font war on this blog, so please spare me the hate mails! 😉