XQuartz disk image won't open
I'd say the file downloaded is corrupted, you should be able to open the .dmg by simply double clicking it (I downloaded XQuartz a couple of days ago and could open the .dmg without a problem).

Unfortunately, the download page doesn't provide a checksum to check against, so I'll provide the one I generated:


  98b2ca8580046d5b91250c5a244c8182437dc9d7


This checksum uniquely identifies the XQuarz .dmg.

You can generate a checksum of XQuartz on your Mac like this (from http://support.apple.com/kb/HT1652):


Open Terminal (in /Application/Utilities)
Type:

openssl sha1 ~/Downloads/XQuartz-2.7.4.dmg 

(adapt to the path where XQuartz-2.7.4.dmg was downloaded)
Compare the number you get with the number above. If they are different, the file is corrupted.