High level runes from diablo 2.
Seriously, while gold may have been the currency with the NPC's..........HLR's on the other hand were much rarer and more valuable. The entire online community was based on HLR's
You could throw them in sockets on weapons and make rune word combinations which created really fucking badass weapons that absolutely destroyed anyone wanting to pick a fight with you.
The things were so fucking rare, that professional "boss runners" (god-like players whose only mission was to kill the bosses over and over and as fast as possible to collect the loot) came about, and started selling the HLR's for real life money through websites.
That, and pylons. YOU MUST CONSTRUCT ADDITIONAL PYLONS.