Turn your iPhone into a VoIP device and make free calls over Wi-Fi. Fring for iPhone lets you do just that.

Available for free at Apple’s App Store (part of iTunes), fring lets you call other “fringsters” or other IM buddies using Skype, MSN Messenger, Google Talk, and ICQ. Other IM clients are supported, too, such as AIM and Yahoo -- but for text chats only. (Apple iPod touch owners can also install and use fring for IM chats and file transfer). You can also add Twitter to fring in the same way you’d add a VoIP service: simply type in username and password.

The color-coded buddy list shows you who’s available on the main fring screen: Green is available, orange is busy, and grey is offline. To receive a fring call, the nickname of a friend will appear on your iPhone screen, and then you tap Accept to begin the conversation.

Based on our testing, audio quality was decent when calling Skype friends on a PC. We experienced a bit of an echo on one of three calls we made. It’s not free, but fring users can tap the Dialer tab to bring up a soft keypad and dial a landline or cell phone using SkypeOut or one of hundreds of SIP-based providers, including VoIPCheap, Eutelia, GizmoProject, and SIPNet.