The IBM JDK offers performance benefits over Sun's JDK, which is especially useful when playing around with, say, the Scala interpreter. Installing it on Ubuntu can be a little tricky, but here's how you do it:

1. Download the IBM JDK version 6 here.
2. Install fakeroot and java-package with sudo apt-get install fakeroot java-package.
3. Edit /usr/share/java-package/ibm-j2sdk.sh and add this on line 28:
   
     "ibm-java-sdk-6.0-1.0-linux-i386.tgz" ) # SUPPORTED
         j2se_version=1.6.0${revision}
         j2se_expected_min_size=82 # 85983232 bytes
         found=true
         ;;
    
4. Navigate to the directory where the JDK was downloaded and execute fakeroot make-jpkg ibm-java-sdk-6.0-1.0-linux-i386.tgz.
5. Install the debian package that was created.
6. Type sudo update-alternatives --config java to choose to use the IBM JDK.

Note: Previously, I had always installed JDKs manually, but if you want to use the JDKs that come in the repositories, make sure to pick up libreadline-java and libjline-java to make it easier to work with the scala interative shell.