Eagle is a good tool for schematics entry and PCB layout, as mentioned before. It is cross platform and works under Linux, Mac OS X and Windows.

However, if you use the apt-get install method, the version might not be the latest and greatest. For example, Ubuntu 12.04 repositories currently host Eagle version 5.12.0, while the latest available version for download is 6.5.0.

This is how to get the latest version:

Go to cadcoftusa download website here and download the latest version for Linux, for example this file:

wget ftp://ftp.cadsoft.de/eagle/program/6.5/eagle-lin-6.5.0.run

Make it executable:

chmod +x eagle-lin-6.5.0.run

Then run the installer:

./eagle-lin-6.5.0.run

and answer all the questions as needed, for example, the directory where you want it installed. Should you want to place it in the /opt/ or other global location, you may need to prepend the installation program with sudo

Also, you can change the license options later.