The iPhone 3GS can be controlled by your voice, and its voice recorder app can be used to take audio notes or record whatever is going on around you. But all of this is done via the iPhone's decidedly underpowered mic. If you want to use these features on a more-than-casual basis, you might want to get an upgrade.
The Flexible Mini Capsule Microphone claims to offer 10 times better audio reception than the internal mic built into your phone. It'll let you bring the audio quality up a notch when recording video with audio or just audio, and it'll only set you back $14.