There's also the Orca screen reader for GNOME. And I believe TalkBack for Android and ChromeVox for Chrome OS are both open source.
If you don't care so much about open source, as I said elsewhere on the thread, Mac has VoiceOver built in (Command+F5 to enable), and Windows has Narrator (Control+Windows+Enter to enable on recent versions).
Disclosure: I'm a dev on the Narrator team at Microsoft, but I'm posting on my own behalf here.