Cheveron WP7 is the user friendly Windows Phone 7 Jailbreak.
If you want to jailbreak your windows phone 7 (or unlock wp7 device) so that you can install apps without needing windows marketplace, follow this step by step guide.
Please note, the jailbreak / unlock process is completely reversible. The windows phone seven unlocker software Chevronwp7, once unlocks your wp7, the next time you connect your omnia7 to the pc via usb port, it shows Relock button to restore your phone back to it’s original state as it was before unlocking.
Note: According to Microsoft, Unlocking your windows phone 7 with chevronwp7 may void the warranty, disable phone functionality, interrupt access to windows phone 7 series or render the phone permanently unusable. 
Proceed to the unlock at your own risk.
Following are the simple steps needed to unlock your windows phone seven device but before proceeding, read all the steps once to make sure you understand how to do all of them to avoid any complication in the middle of the process.
1. Install Zune desktop
Download and install latest version of Zune desktop software for windows from here.
2. Edit the registry key
Goto Start > run
Type regedit and press enter
Goto: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsPhone\ProxyPorts
In the right hand window, right click in empty window and create new DeviceReg giving it a dword value 000069c5
Writing all this in technical words:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsPhone\ProxyPorts]
“DeviceReg”=dword:000069C5
3. Connect your wp7 device to pc
Connect your windows phone seven (omnia 7) to pc via usb.
4. Sync your device
Launch the Zune software and sync your windows phone seven device.
Make sure your device has a full sync relationship, not Guest relationship with zune software.
5. Download and run Chevron wp7
Download and run the windows phone seven unlocker app – Chevronwp7.
6. Check for pin lock
Make sure your device is not locked with PIN code
7. Install Certificate on phone
Make sure that the device isn’t PIN locked. Then install the certificate on your wp7 using internet explorer from http://chevronwp7.com/cert options.
8. Unlock wp7
Once done with certificate installation, click on the Unlock button.
Your windows seven phone is now unlocked and you can install or sideload any third party wp7 app without requiring a marketplace developer account!
The best thing about this “unlocker / jailbreak” is that it is completely reversible. If you connect your phone with PC again and run ChevronWP7 app, you will notice a “Relock” button which will allow you to relock your phone to its original state.