There are plenty of third-party apps in the App Store that offer additional options (or add-ons) for text messaging. One of my favorites is TextPics (or there is a free version available here). However, did you know that there is a plethora of standard emoticons available through the iOS 5 native software?
This feature has not been widely advertised, and is buried deeply within the settings menu. To be more specific, it has been camouflaged as an "international keyboard" under the language "emoji."
For full instructions, follow this easy how-to that I posted over at WonderHowTo.com.
Enjoy your new emotion-filled text conversations.