The following link provides a list of some of the available systems: Thinkengine Networks, Inc. Several are also referenced below, some are not.
Information on Loquendo TTS, including technical specifications, is available at: Loquendo.com. There is an interactive TTS demo on the Loquendo home page.
Information and specifications are provided on the AT&T Web site for Natural Voices Text-to-Speech Engine.
One comment about Nuance's RealSpeak was that the quality of the speech from Nuance is parallel to or better sound than Loquendo. The site includes an interactive demo. Rhetorical, a high quality TTS program, has "become" Nuance. It has been reported that rVoice (Rhetorical) is being discontinued, and will be replaced by the next version of RealSpeak. The new version of RealSpeak will include conversion protocols to enable it to replace rVoice with no change to application which may have been developed specifically for rVoice.
There are samples including multiple voice options for AT&T Natural Voices, NeoSpeech, Cepstral and RealSpeak.
Capital Accessibility will be offering IBM's text to speech for Linux, formerly known as Viavoice, in the very near future. TTSynth will be available in about a dozen languages:
Acapella group (was "Babel") and SVOX are two others that may be worth reviewing