If you are in love with fonts available from Microsoft of course you could install
msttcorefontsat command line or simply add fonts from fonts folder in your Windows partition.
From Ubuntu guide you could also add extra fonts repositories
deb http://www.telemail.fi/mlind/ubuntu feisty fonts
deb-src http://www.telemail.fi/mlind/ubuntu feisty fonts
sudo apt-get updatethen reconfigure the fonts by
sudo apt-get install libfreetype6 libcairo2 libxft2
gksudo dpkg-reconfigure fontconfig-config
gksudo dpkg-reconfigure fontconfig
my selections were 'Native', 'Automatic', 'No bitmapped fonts' during the interactive configuration. After that reboot the system or press Ctrl+Alt+Back Space.
I have also added some fonts from my Windows XP installations.
If you are really hungry for more fonts you could add 3rd party fonts by
sudo apt-get install cabextract gsfonts-other msttcorefonts
t1-xfree86-nonfree ttf-f500 ttf-isabella
sudo apt-get install ttf-larabie-deco ttf-larabie-straight
ttf-larabie-uncommon ttf-staypuft ttf-summersby
sudo apt-get install ttf-ubuntu-title ttf-xfree86-nonfree
At the end I jumped from 102 to 496 fonts available to me. I would say fonts look 95% like OS X now.
Last but not least the font size still looks odd in firefox they are just too thick and bold. From this thread it seems it has nothing to do with system font, I guess tweaking firefox config is a solution.