Jonathan Mitchell was found dead in the kitchen of his western Sydney house after attending a music festival in December 2003.
The 28-year-old had left the Homebake festival at The Domain and met up with his brother at a Sydney pub, before returning to his Toongabbie home about 7pm.
He was bashed with a cricket bat and stabbed in the neck, severing his spinal cord.
The bat was later found in a gutter near the house.
An inquest concluded he was likely attacked by a person or persons unknown to him.
Police are hoping a $500,000 reward for information will help flush out Jonathan Mitchell's killer. (Supplied/AAP PHOTOS)
The $500,000 reward announced on Friday increases a $100,000 reward for information in place since 2009.
A bedroom in the house was ransacked, with drugs stolen, police said when announcing the previous sum.
It was a "known drug house", but Mr Mitchell was not believed to have been involved in drug dealing, police said.
He was described as a "nice guy, everyone's friend, (who) would give his shirt off his back to help anybody".
NSW Police have offered rewards relating to dozens of deaths, disappearances and other unsolved cases.
They include 40 appeals for information with rewards of $1 million on offer.
Among them are some of the nation's most high-profile unsolved cases, including the disappearance of toddler William Tyrrell, a fire at Luna Park's ghost train ride that killed six children and a man, and the murders of three children in a four-month period at Bowraville.
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