[update] This process has changed in Ubuntu 8.04 and higher. See How to Install Fonts in Ubuntu 8.04 for updated instructions.
Want to add some fonts you downloaded to your Ubuntu system? The Nautilus file manager includes an interface for adding fonts. Using Nautilus is the easiest way to add fonts to Ubuntu.
Press Alt-F2 to open the run dialog. Type the following into the text box and click “Run”.
The window that opens allows you to copy and paste new font files to install them.
For applications to see new fonts they needs to be restarted. If you are looking for a font, I would recommend Dafont.