Overview
RPG Maker 2000, originally released in Japan as RPG Tsukūru 2000 (RPGツクール2000) on April 2000, is one of the many versions of RPG Maker available. RPG Maker 2000 and 2003 are the most commonly used engine in Yume Nikki Fangames, likely due to Yume Nikki's usage of RPG Maker 2003. However, RPG Maker 2003 has become unavailable to Japanese users, leaving 2000 as the norm for Japanese fangames.
More information can be found on the Wikipedia page.