If you log into GNOME, you can use Control Center / System Settings to enable Orca. I've played with Orca in the past, and found it very impressive.
Yesterday, I used it to read out a plain text ebook in gedit. But my netbook didn't have room for a GNOME environment as well as my default KDE.
Here's how I enabled Orca for GNOME programs, despite not using a GNOME desktop.
0. Install orca. (ubuntu package gnome-orca). You may also need to make sure you have a speech synthesizer installed, eg. espeak
1. Enable accessibility.
If you don't have the GNOME control center installed, it's still possible to do this manually.
I used gconf-editor to enable the setting /desktop/gnome/interface/accessibility. I suspect that's a bit out of date though. I've also seen this suggested:
gsettings set org.gnome.desktop.interface toolkit-accessibility true
2. export GTK_MODULES=libgail
You need to run this command from a terminal, and then run your GNOME applications from the same terminal.
If you're logged into GNOME, this part is taken care of automatically.