Where is Java installed in OSX Mavericks?
George Garside's path is correct. Latest JDK released today from Oracle. I advise downloading directly from Oracle's Site. Use the symlink to create your JAVA_HOME variable. 
Add the following to your .bash_profile or .bashrc profile. 

export JAVA_HOME=$(/usr/libexec/java_home)


Just downloaded the kit and added the above to my .bash_profile.
As you can see, variable is now clearly defined:

Macbook-Pro:~ njboot$ echo $JAVA_HOME


Returns:

/Library/Java/JavaVirtualMachines/jdk1.8.0_20.jdk/Contents/Home