Currently there are three major players: iPhone (iOS), Android-based, and BlackBerry.
While connecting one-to-one is the basic feature, connecting as a group is quite limited. Eventhough Facebook runs on all platform, it doesn't have group-chat feature (yet). Some popular messengers (Yahoo!Messenger, MSN) also available in all platforms, but they also don't have group-chat feature (yet). They have conference feature, but you should invite the members everytime need to chat, not permanent. One that have this group-chat feature is BlackBerry with its BlackBerry Messenger (BBM), but it's ONLY among the BlackBerry users, and not everyone fans of BlackBerry...
PingChat! is one of the apps that runs on all those three platforms (iOS, Android, and BlackBerry) and have group-chat feature (as of the time I write this blog, the group chat max. 15 members, hopefully they will increase it in the future). Beside that, it also be able to send picture, audio, and video. The interface is also nice and looks professional. It uses the push notification method, so it works like SMS. The best of all, it's FREE.
PingChat! is always Online. There is no concept of signing in or signing out in PingChat!, unlike other instant messaging platforms. Once you install PingChat! and register your PingChat! ID, you will be always be online. This is great because you don't have to wait for your contacts to sign in to be able to chat with them.
Interested to know in detail? Go to PingChat website here.