Microsoft Text-to-Speech Engines for SAPI5
     Microsoft Mary, Mike and Sam 5.0 (68MB)  Hot!

CMU Kal Diphone Voice for SAPI5
     CMU Kal Diphone Voice (2.61MB)
          Usage: Unzip to a folder and run setup.bat.
          For more information, visit Flite Home.

L&H TruVoice TTS Engines for SAPI4
     American English (0.99MB)  Hot!
     British English (2.54MB)  Hot!
     Dutch (2.58MB)
     French (2.24MB)
     German (2.18MB)
     Italian (1.97MB)
     Japanese (3.00MB)
     Korean (3.03MB)
     Portuguese (2.39MB)
     Russian (2.85MB)
     Spanish (2.36MB)

Listen to some Microsoft and L&H voices for SAPI4:  

Some voices are also installed by default with certain software programs:
* Microsoft Sam 5.0 is also shipped with Windows XP.
* L&H Michael and Michelle 5.0 are shipped with Win 2000 and Office XP.
* Microsoft TTS 4.0 voices are also shipped with Encarta 99 and higher.

AT&T Natural Voices for SAPI4 and/or SAPI5
     Buy Natural Voices from HomeSeer, an AT&T-licensed distributor:
     Choose from a Complete List of Natural Voices  (free test listen available)
     Choose the Mike and Crystal Voices Only  (free test listen available)

Digalo Text-to-Speech Engine (Crash after trial expired, details)

Elan Speech Engine

Eloquent Technology ETI-Eloquence

IBM ViaVoice Outloud (Good voices, 90-day demo)