Some developers over at Apple have hidden more than 400 emoticons and other symbols in iOS3 and iOS4. To activate them, just download the App “Emoticons” or “Emoji Enabler” from the App Store (they are free) and follow the instructions. You might have to reboot your iPhone. Afterwards the smileys will show up as an additional keyboard in your normal text writing window. The secret emoticons can only be used between iPhone users.
There are several hidden smileys in Skype you can use when chatting with others. Just type the name of the emoticon in brackets in your chat window.