Google Maps Navigation will be part of Android 2.0, and Android 2.0 is expected to come to most—if not all—Android handsets on the market eventually, so given how complicated this hack is (fairly!), and that it requires a rooted phone, it'd be prudent for most folks to just wait this out. That said, there's no saying how long it'll be until Eclair seeds out to older handsets, and there's no guarantee that the G1—and old fart in its little corner of the universe—will ever get it. Anyhow, here's what you need to do, from XDA:
Make sure you have CyanogenMod 126.96.36.199, by the way.
1) Unzip zip to desktop
2) Run Part 1.bat
3) When prompted to press any key, do so; the phone will reboot
4) When your phone is running again, run Part 2.bat
5) When prompted to press any key, the installation process is done.
6) Make sure GPS is enabled
7) Open Maps and press OK
8) Press Menu
9) Press Directions
10) Enter an end point
11) Click Go
12) Just under "Show on map", click Navigate.
13) When prompted to install the voice codec, do so.
And there you have it: Free, not terrible turn-by-turn navigation for your G1, without Android 2.0. Early reports say it runs well; let us know how it goes for you in the comments. [XDA via
Engadget Mobile via IntoMobile]