Skype - free IM & video calls I love using Skype on my laptop, and the app is pretty cool too. However, if you don't have a front and rear facing camera, you can't do video chat. If they fix this, 5 stars all the way!
Facebook Messenger If you have Facebook on your android, messenger is a must have. I hope the next version allows saving of photos, however.
WhatsApp Messenger A really great app for quick messages if you don't like SMS, or the limitations of SMS. Especially useful for sending messages to groups. Also, messages will be saved if you turn off your phone and go offline.
KakaoTalk: Free Calls & Text Great for anyone that communicates to friends internationally, or with groups. No need to worry about text overages, and I really dig that you can send audio recordings. It's fun that you can also make a call and use a voice filter to freak people out!
GO SMS Pro Amazingly cool app that controls all of your messaging, including chat and email support along with SMS. You can totally customize the appearance as well, which is super awesome.