Simon is a very nice KDE4 project with the goal of developing a speech recognition application and it currently works pretty well by letting the user record certain words and execute various commands given via a microphone. Since currently it is not included in the Ubuntu repositories, here are instructions on getting it up and running in Jaunty.
First of all, open up Konsole or GNOME Terminal and install the package flex, needed by simon:
sudo apt-get install flex
Next, download the Ubuntu DEB package from kde-apps.org page and install it as:
sudo dpkg -i simon-0.2-Linux_i386.deb
Or whatever version you downloaded. This should be it, run simon by typing Alt+F2 and typing simon followed by Enter.