Handcent SMS is an awesome, free Android app, being part of the category Communication Apps with subcategory SMS & IM.
imo was originally a messaging app that could manage a lot of different social networks and chat apps.
BlackBerry Messenger (BBM) is an instant messaging application.
Hike Messenger is messaging app for iPhone that lets you chat for free with any other user of the service.
SOMA Messenger is an instant messaging app with two standout features: privacy, and high-quality voice and video calls for free.