The new Voice Search application on Android, starting today, lets you do tasks with just the voice search button and speaking what you want to do. The first example: texting a person by saying their name and the message.
Second example: Calling a business you don't have a number for, but know the name of. It uses Google Maps to find the business and route the number.
Another one: Navigation. Say "Navigate to Rockefeller Plaza", and it will bring up the navigation app and drive you to the destination. Then, finding music on your phone, which will go out on the internet and grab music to play back on an app on your Android phone. Once you find a result, it'll come back and show you which apps support this music feature, and you can pick one to play (like Pandora).
These voice actions are "fast and easy to use", but you don't have to do everything at once, like send an email in its entirety from the start screen. You can add more text to an email, for example
It also does menial tasks, like "set alarm for 8:30 AM," or "Go to Wikipedia."
Another cool use case: Note to Self. Where you have a fleeting thought, you say "Note to Self: Blah blah blah" and it'll make an email to you to remind you of what you just said.
There are ten of these Voice Search features total. It is available today on Android 2.2 devices. Search for "Voice Search" in the market.
Here's the big list of Voice Actions:
send text to [contact] [message]
listen to [artist/song/album]
send email to [contact] [message]
go to [website]
note to self [note]
navigate to [location/business name]
directions to [location/business name]
map of [location]