Xboard is a graphical interface to play chess against the computer. It does not really include any engines, however they can also be freely obtained and installed. Recent versions of xboard, such as the one included in the Lucid repositories, also can use engines that follow the UCI protocol, which is becoming more popular than the native xboard protocol.
In Ubuntu, the command:
$ sudo apt-get install xboard
You can install xboard-protocol engines by installing the packages:
and then use the command:
$ xboard -fcp crafty -tc 10 -inc 0
to play against crafty a game with 10 minutes per side, and no time increment.
There are UCI engines in the following Ubuntu packages:
Unlike the engines following the xboard protocol, one opening book works for all UCI engines. I personally like the book performance.bin, which you can find here.
Then you can use the following command to play against Toga:
$ xboard -fUCI -fcp toga2 -usePolyglotBook true -polyglotBook ~/performance.bin -tc 10 -inc 0
(assuming you put performance.bin in your home folder).