To play on a modded Xbox 360 (JTAG/RGH), you can download the game in ISO or Games on Demand (GoD) formats from various community sites. For a stable experience, it is highly recommended to use the GoD format , as extracted ISO files often suffer from freezing and fatal crashes. Installation Guide for JTAG/RGH
The "free" part of your query points to downloading the game files rather than buying a disc. There are several sources where you can find these files, often in ready-to-play formats.
JTAG and RGH are hardware-level exploits that allow an Xbox 360 to run unsigned code, meaning you can play backups directly from a hard drive without a physical disc. For Warfighter
There are several benefits to playing Medal of Honor Warfighter on a JTAG or RGH console. Some of the benefits include:
Using JTAG or RGH can brick your console if not done or used improperly. There's also a risk of online bans on games that detect such hacks.