If you're one of the lucky ones to have scored an invite to Google Voice, check out VoiceCentral. It's a very slick application which integrates well with the iPhone, making it possible to make calls through your Google Voice (or GrandCentral) account.
Why would you want this? My primary reason for wanting an application for GV is so that I can make calls and have my Google Voice number appear on the Caller-ID.
VoiceCentral makes this a simple process. When you first install the app, it asks you to select your iPhone number on the Settings page. When you make a call through the app, it happens in two steps: 1) your iPhone rings (this is Google Voice's servers calling your iPhone), 2) your call is connected.
Why is this important? If you end up leaving a message for whoever you are calling, their voicemail system may give them the option to call you back at the number you called from. Our landline phones also make it easy to return calls from the caller-ID list. As a Google Voice user, I want those calls to go to my Google Voice number, not my iPhone.
If you're a fan of Google Voice, you know that having a central place for all of your voicemails is great. Google Voice will transcribe those voicemails (my favorite feature, although sometimes having a computer try to decipher my friend's "late weekend night" messages does lead to some interesting "translation issues").
VoiceCentral will let you access not only those voicemails (in a style very similar to iPhone's own Visual Voicemail) but if you tap on the arrow, it will also show you the transcription. You can even read the message without listening to it, which is especially handy for checking messages when you are in a meeting, a movie theater (oh, like you've never!), or anywhere loud enough that you might not be able to hear it. The transcripts can also be copied (on iPhones running OS 3.0 or later, of course) and pasted into notes or emails if needed.
As you'd expect, VoiceCentral also integrates with your existing iPhone contact list, and there's also a keypad which allows you to make calls or send SMS.
The "Send SMS" feature has had some issues, but Riverturn has a newer version which fixes the problem. As you are no doubt used to hearing by now, they are waiting on Apple to approve the newer version. It's also important to note that there is no way to designate any application except the built-in Phone app as the default, so any phone numbers which you tap from email messages or web pages will open in the built-in app, rather than in Skype or other telephony apps.
Google appears to be readying its own GV iPhone app for release, although it hasn't made it into the App Store yet. Anyone familiar with the approval process knows that could mean it is hours, days, or weeks away from being released. The Blackberry and Android apps are already available.
Is $2.99 [iTunes Link] too much to spend on an app when there may be a free alternative right around the corner? Not for me. In fact, the only thing which keeps this app from replacing the native Phone app on my dock is the lack of a "Favorites" tab, which I use for nearly all of the outgoing calls I make on my iPhone.