As for the iPhone itself, the most obvious addition to OS 3.0 beta 4 was the voice recorder, and I absolutely live it.
The application appears as any other on the iPhone's desktop. It's a little 50s-era microphone. Open it, and the microphone fills most of the screen, making it pretty obvious what the application is for. There is a voice signal strength meter (looks like an analog volt meter) that shows you how loud you are.
Press the small record button to start a recording. While recording, you have the option to pause or stop, as on most phones with voice record capabilities. Once you stop recording, it wraps up and stores the audio file. You can access a list of your audio files by pressing a small button on the right side of the microphone.
The voice files can be shared via email or MMS. They can also be played back over the speaker.
Very cool. I can't say it is any more feature-rich than other voice memo recorders, but it's a lot more fun to use.