X11 X11 is a windowing system used by some Unix software to provide a graphical user interface under Mac OS X. If you use X11, chances are you'll know it. On the other hand, at approximately 64MB, it's worth installing just in case you ever come across an app that needs it.
Rosetta Disabled by default, Rosetta is the behind-the-scenes program that lets you run many PowerPC applications on Intel Macs. If there's a chance you have older Mac software that never made the jump to Intel code, you should install Rosetta--although, as with printer drivers, Snow Leopard is forgiving here (see below).
QuickTime 7 Snow Leopard includes a new version of QuickTime that claims to offer many improvements over QuickTime 7. However, the new version may not support some older media formats that QuickTime 7 handled with aplomb (see this Apple support document for details). Check this option to install QuickTime 7 for backward compatibility. If your Mac has an active license for QuickTime Pro 7, this option will be enabled by default.
Even though you have all these options to consider, don't spend too much time worrying about what to install: unlike Leopard and earlier OS versions, Snow Leopard can automatically download and install some software on demand. For example, if you didn't chose the All Available Printers option and you later try to use a printer that's not supported by your original installation of drivers, OS X will connect to the Internet and download drivers as needed. Similarly, if you didn't install Rosetta or QuickTime 7, and you later try to open a program or media file that requires that software, OS X will offer to download and install it on the fly (assuming you have an Internet connection, of course).
In addition, if you forget to install something the first time, or if you're having problems with an application and you want to get a fresh copy, you can easily install it from the Snow Leopard DVD by double-clicking on the Optional Installs.mpkg package inside the Optional Installs folder. After the installer launches and checks your drive for Snow Leopard, you can choose to install Rosetta, QuickTime 7, additional fonts, printer drivers, language support, iPod support files, X11, or any of the stock OS X apps (Address Book, iCal, iChat, iTunes, Mail, Dictionary, or Safari).