2D fighting fangames for Key and Leaf, respectively. I believe the versions most often played are Eternal Fighter Zero: Memorial Edition and The Queen of Heart '99.
QOH contains characters from various Leaf games (Shizuku, Kizuato, To Heart, etc.), and EFZ contains characters from early Key and pre-Key Tactics (MOON. to AIR, though most characters are from One or Kanon), as both companies have rather large fanbases. EFZ in particular is known for being by Tasogare Frontier, the same people that made Touhou 7.5 and 10.5.
Neither game is exactly new, so they may have their fair share of issues when running on a newer version of Windows. Using 7 myself, I have palette issues galore and can't move QOH99 past the title screen. But in any case I find fangames such as these interesting now and then and wanted to see if anyone else had possibly heard of them.