avant window navigator
But have a look at SuperKaramba and things of such ilk. In the open source world, if you are after a close-source functionality, you are usually being too limiting.
There are lots of speech stuff in OSS/FS. You'll have to outline the actual functionality involved. If you want to know if you can use the exact speech sounds included in mac - read the license that comes with them. If it ain't compatible, then any alternative will sound different.
Of course, you could look at: