As game development industry is opening more towards small publisher studios and independent developers. Almost every leading game developer studio is coming up with open source SDK version which carries source used for commercial games.
Makers of unreal tournament released the development kit for free.They offer this SDK free and for almost any type of game development usage be it commercial or presentation or for education,personal,collaborative project.

System requirements are higher so if you’re with limited system resource then this SDK is not for you.
Minimum:
* Windows XP SP2 or Windows Vista
* 2.0+ GHz processor
* 2 GB system RAM
* SM3-compatible video card
* 3 GB Free hard drive spaceRecommended for Content Development:
* Windows Vista 64 SP2
* 2.0+ GHz multi-core processor
* 8 GB System RAM
* NVIDIA 8000 series or higher graphics card
* Plenty of HDD space
Check the UDK site here.