Microsoft’s beautiful “Core Fonts” used to be available for (free) download at http://www.microsoft.com/typography/fontpack/default.htm.
Unfortunately, these have now been removed—see http://typographi.com/2002_08_13.php#000428.
Fortunately, the licence agreement under which they were originally distributed makes it possible to make these fonts available for download here.
To unpack the *.exe files on OS X, you’ll need cabextract. (There’s a MacPorts version as well.)
198383 2002-08-14 08:30 andale32.exe 554208 2002-08-14 08:30 arial32.exe 168175 2002-08-14 08:30 arialb32.exe 246007 2002-08-14 08:30 comic32.exe 646368 2002-08-14 08:31 courie32.exe 392439 2002-08-14 08:31 georgi32.exe 173287 2002-08-14 08:31 impact32.exe 661728 2002-08-14 08:31 times32.exe 357200 2002-08-14 08:31 trebuc32.exe 351991 2002-08-14 08:31 verdan32.exe 185071 2002-08-14 08:31 webdin32.exe