Loquendo has been mentioned before (oddly without much response) but...
Actually, these will replace the Text-To-Speech engine used so they'll affect anything that uses TTS. Be sure to read the requirements. IIRC, Froyo is required.
Loquendo TTS Susan
Loquendo TTS Susan - Android app on AppBrain
IMO this engine had a better voice listening to the samples but it was quite choppy in Google Maps Navigation. This engine is not free.
SVOX Classic TTS
Classic Text To Speech Engine - Android app on AppBrain
Currently there are more voice options for this engine. The engine is free but the voices are not. Note that the voice data for each voice takes up quite a bit of internal storage and does not currently support app2sd (though it claims it installs voice files to the SD card). Make sure you open the voice files via the Market app or the notification bar after they are downloaded and installed to complete the setup.
I'd say that either of these engines sound better than the stock Pico TTS engine. Make sure you read the instructions and follow them completely.