Possible engines might be:
- MARY (another project from the DFKI)
- eSpeak - this is more or less the Linux default
- flite, which is Festival lite.
- And Festvox, which is the full Edinburgh/CMU Festival system.
I can only imagine that Vocaloid is a unit synthesizer with a large database; the output is pretty natural-sounding, in contrast to the state-of-the-art of truly synthetic speech. It would be a lot of fun to play with this stuff, especially in the context of music.