CyanogenMod is a free, community built distribution of Android 2.2 which greatly extends the capabilities of your phone.
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This is a pure, working AOSP build which you can use without any type of Google applications. I found a link from some other project that can be used to restore the Google parts, which can be found below or elsewhere in the thread. I’ve still included the various hardware-specific code, which seems to be slowly being open-sourced anyway.
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DISCLAIMER

While this build is heavily optimized, it is also capable of pushing your G1 much harder. I am not responsible for bricked devices, dead SD cards, thermonuclear war, or the current economic crisis. Please do some research if you have any concerns about features included in this ROM before flashing it! YOU are choosing to make these modificiations, and if you point the finger at me for messing up your device, I will laugh at you.
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• Can be used with or without Google applications
• Kernel 2.6.34 with Pershoot’s tweaks
• ADWLauncher is the default launcher!
• Complete music app overhaul by Eliot Stocker
• Clean shutdown/startup system
• Built in USB & Bluetooth tethering
• ARM NEON optimizations from 0xdroid and Qualcomm
• Includes BusyBox, htop, nano, powertop, openvpn, bash, and all engineering commandline utilities
• FLAC audio support from Kenny Root
• MMS enhancements from ruqqq (landscape keyboard)
• Perform zipalign-on-install of applications
• DeskClock tweaks- longer autosilence and “require unlock” option
• Customizable Nexus live wallpaper by Chris Soyars and me
• Full color trackball notifications (from Google)
• OpenVPN integration thanks to James Bottomley
• Many other bugfixes and tweaks from AOSP contributors
• Tons of work on the build system by Koush and Chris Soyars
• See the full CHANGELOG for all credits and details

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All source code is available at the CyanogenMod Github! Send pull requests with your changes!

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HOW TO REPORT BUGS OR PROBLEMS?
- Was it a hard reboot? Get me the file “/proc/last_kmsg”.
- Was it a soft reboot or a “boot loop”? Run “adb logcat” and get me the full output.
- Pastebin links preferred
- Please use the issue tracker whenever possible!
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MILESTONES
1st Android Rom Developer with a Blog, Forum and Wiki.
1st Android Rom Developer with a Facebook Page.
1st Android Rom Developer with a fan base of over 30,000.*
1st Android Rom Developer with a Rom Updater compiled together with a Theme Updater.
1st Android Rom Developer with a an IRC Channel.
1st Android Rom Developer with a dedicated YouTube Page for Tutorials.
1st Android Rom Developer with BFS included in the ROM.

*Based on CM-Updater statistics.

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WEBSITE INFRASTRUCTURE
The Website, Forum, WiKi and YouTube pages are all developed and maintained by SatanR1 of MetServe Enterprises. Any problems with the website infrastructure (not the rom or it’s development) should be made towards him. You can email him directly here or you can visit the company website here.