Are there good jailbreak apps that will improve the performance of iPhone 3G?

Not a jailbreak solution, but... turn off SpotLight.

Go to Settings > General > Home > Search Results, and turn everything off.

This should result in a noticable speedup.

NB - these instructions apply to iPhone OS 3.0, link to instructions for iOS 4.x at the bottom

You can run your iPhones processor at a higher-than-factory clock speed which should in theory make it faster. However, it may cause the iPhone to run too hot so it's something to be aware of. The steps are below:

1. Jailbreak the iPhone
2. Use SSH to navigate to /System/Library/System/System Configuration/PowerManagement.bundle/
3. Rename file com.apple.SystemPowerProfileDefault.plist to something else
4. Restart your iPhone

Your iPhone will now run at the default speed for the processor it is built on. You may experience instability as the iPhone was designed to run at a speed lower than the native speeds of the processors it's built on so there's a major health-warning on performing this step.

iOS 4.x instructions thanks to @mankoff for his comment pointing out that the instructions had changed with the new iOS version. I expect these will change again in future so be sure to check Google or the search box on this site for latest methods.