I'm excited to say I've started writing a personal assistant for the Mac. It's called Samantha, and it's very similar to Siri on iOS.
For this to work best you should have OS X 10.7 or later with Speech to Text enabled. If you want it to sound like Siri, select Samantha as the Text to Speech voice.
Some commands (as of beta versions)
-Play Alone Together
--What time is it?
--Google Voodoo Donuts
--Toggle music (plays or pauses the music coming through iTunes)
--What model mac am I using?
And much, much more.
I've also added the ability to write extensions for it, so you can add anything else I haven't. Just say "Example extension" and you'll get more information.
If you like this or have some suggestions, please let me know.