Go to the icon 'Messages' on your iPhone, enter the message you want and add the contact you want to send the SMS.
As you can see, if your contact is using an iPhone or iPad with iOS 5, the color of the icon 'Send' will be blue and the message field will appear as 'New iMessage'. Therefore, you are sending a free iMessage message from your iPhone.
However, if your contact does not have iMessage, icon 'Send' will be green and the message field will appear as 'New Post'. Watch this little detail and save yourself future scares your phone bill.