Some of the main features of eSpeak are:
A command line tool for Linux and Windows
Speaks text from a file or from stdin
Shared library version for use by other programs
SAPI5 version for Windows, so it can be used with screen-readers and other programs that support the Windows SAPI5 interface.
Ported to other platforms, including Android, Mac OSX etc.
Several voice characteristics to choose from
speech output can be saved as .WAV file
SSML (Speech Synthesis Markup Language) is supported partially along with HTML
Tiny in size, the complete program with language support etc is under 2 MB.
Can translate text into phoneme codes, so it could be adapted as a front end for another speech synthesis engine.
Development tools available for producing and tuning phoneme data.
To install eSpeak in Ubuntu based system, use the command below in a terminal:
sudo apt-get install espeak
eSpeak is an old tool and I presume that it should be available in the repositories of other Linux distributions such as Arch Linux, Fedora etc. You can install eSpeak easily using dnf, pacman etc.
To use eSpeak, just use it like: espeak and press enter to hear it aloud. Use Ctrl+C to close the running program.
There are several other options available. You can browse through them through the help section of the program.
GUI version: Gespeaker
If you prefer the GUI version over the command line, you can install Gespeaker that provides a GTK front end to eSpeak.
Use the command below to install Gespeaker:
sudo apt-get install gespeaker
The interface is straightforward and easy to use. You can explore it all by yourself.
While such tools might not be useful for general computing need, it could be handy if you are working on some projects where text to speech conversion is required. I let you decide the usage of this speech synthesizer.