there very little good 'voice synthesis' about really, but there are few that are ok, and combined with Abletons Formant TimeStreching (Complex Pro) and a few FX you can get them sounding ok, MS Sam isnt far off the best out there
I would look for Vocaloid or the cracked/hacked version Pocaloid, which contains several different voice types.
But it's probably more useful for making singing vocals.
Vocaloid's most popular voice will be the Japanese voice pack called 'Hatsune Miku', which in Japan has grown to a full grown pop star.
That while the artist itself is a computer program, the software sings the vocals.
She is 'performing' as a hologram/projected animation now throughout entire Japan and has done two shows in the States.
A absolute outstanding piece of art!
I think this is the closest you can get with Voice synthesis software.
i normally used to just search "text to speach" on google and theres a few free ones on there, wouldnt really reccomend using them in tunes tho, merely to stick over mixes and stuff.
On a similar note there is an amazing piece of software called "EWQL - Symphonic Choirs"
basically you can tell an entire symphonic choir to sing anything and in any key or note via midi triggering. it is quite impressive how well it works.
Gonna have to check this out methinks, jus for fun factor, the symphonic choirs library alone is 40gb and so that always put me off getting it, any ideas how big the library for vocaloid is??? i always thought it would be jokes to make an accapella at 175bpm saying summin like "freeeeeeeekk in the mixxxx in the mix in the mix in the mix" all pitched and melodic. n then jus find some wicked random mixes with it looool!!
I think the complete library from Vocaloid was 21 Gb but a lot of them are Japanese voice packs.
So if you only want the English packs then it would be a lot less. Still about 8 Gb though.
The Japanese packs require writing in phonetic Japanese instead of English.
But still the pack like the Tonio is great, it's a male opera voice pack. Great sound.