You can find many apps in App store to record video with your iPhone 3G. These applications typically cost $1.99, but there is now a free alternative: Record Videos For Free. This is not a lite version and This is a FULL featured application available in the App Store. After filming the video you can export the videos from your iPhone to your PC or Mac via Wi-Fi to transfer. Needless to mention the application contains advertisements which the authors hope to earn their money.
The quality of the videos is moderate, but in comparison to paid video applications for the iPhone 3G fine.
Download: Record Videos For Free (free)