FacePlant brings FaceTime chat status and visual voicemail to the iPhone 4
This free app with a silly name was produced by the team behind the successful 12seconds video app, and it makes FaceTime calls a lot simpler to arrange.
FacePlant shows you when your friends are available for FaceTime, and it lets you leave a video voicemail for them when they're not. Sure, audio voicemail comes with the iPhone, but who else is doing it with video? This idea is novel and pretty cool, even if you wouldn't necessarily use it regularly.
The downside to FacePlant is that your friends also have to have it installed in order for you to see their availability. I definitely think you should start inviting them right away; FacePlant is a good idea that I suspect Apple will eventually copy in some form, and everyone with an iPhone 4 should start using it.
FacePlant hasn't launched publicly yet, but you can sign up to see when it does.