Howtoforge has a nice article on how to install the packages you've installed on another computer. It's written for debian and ubuntu systems, including step by step screen shots for doing it in Ubuntu with Synaptic. Here's how you can do the same thing in CLI(also in that article):
sudo dpkg --get-selections "*" > package_list
sudo dpkg --set-selections < package_list && sudo apt-get -u dselect-upgrade
Oh and from the article's comments: The method only works well if you use the exact same version of Ubuntu on both computers. Let me add that it only works at all if you have the same /etc/apt/sources.list and sources.list.d/* files on both computers.