Google Mobile app

Google Mobile App for the iPhone (free) allows you to easily search by voice (setting turned off by default). Just hold the iPhone to your ear and it will automatically start listening. It seems to work best with common words.
Some features are:

• Saves having to type search queries - just say what you'd normally type into Google
  
• It is location aware so 'pizza' will find local pizza businesses
• Once you say your search query, you can search by just e.g. images