Viber : Free Messages & Calls While the name may not be the best this little app doesn't require registration, uses your existing number, can be used over WiFi or 3G and lots of features!

Qik Video for Sprint For those of you with HTC EVO's (and other devices on Sprints network) this little app is pretty neat. They've recently updated this app and now you can chat with iPhone /iOS users as well!

Skype - free IM & video calls A staple of the VoIP applications this little beauty is great for international calls (cheaper than calling from your cell phone or landline). It does have a few limitations and is a mostly paid product for VoIP calls. Does have a built in IM system which is handy.

fring Free Calls, Video & Text Fring is another staple of the VoIP application pool. This will use your existing IM services (Skype, Gtalk, MSN, AIM, ICQ, Twitter and SIP) to make a "friends list" that you can call. This app works over 3G and WiFi as well. You'll need to sign up for a Fring account.

Acrobits Softphone Acrobits is a fairly new app to the general audience. Used more for business users but has recently added Google Voice integration. Lots of features for audio customization. Works over 3G and WiFi.