XBoard is a chess game for Linux computers. It has been around for a very long time. It's graphical interface is not very fancy and it does not have a trendy look. It's great for playing chess against a computer opponent but it's not that great if you prefer online play. XBoard supports connecting to online servers but it's only possible to do so using command-line parameters.
Features and usability
XBoard's got everything you need and want from a chess program - as long as you are content with playing against a computer game engine. There is no support for connecting to online chess servers in it's interface.
You can select chess engine to play against from the command-line. It will start with GNU It starts Gnuchess if no other engine is specified.
To start Crafty:
xboard -fcp 'crafty xboard'
The -tcp option supports many arguments, for example:
xboard -fcp 'crafty xboard hash=12M hashp=5M'
You can also connect to chess servers using command-line arguments. For example:
xboard -ics -icshost freechess.org
This works, and you can use XBoard to connect to chess servers using command-line arguments just fine. It's just not very user-friendly.
On the topic of command-line switches: There are many more. The xboard manual page is actually very long.
XBoard is a very mature chess program with many "hidden" features. It's manual page is as long as a paper-back novel. And that is a problem. You do not want to install a chess game on some old persons computer and explain how to connect to other players online starting with "This is how you open a terminal and then you type in xboard -ics -icshost and the server". You probably don't want to open a terminal just to get online even if you already know how to use one. And there's no point in doing when you can install Knights or another program which is capable of connecting you with a few clicks.