Shows your current FPS in a little window that you can place wherever you want.
Adds options for coloring, font size, coloring by threshold, turning off the "FPS" text and more.
You can only move the window while the options window is open. Moving is done by either adjusting the sliders or dragging it around with your mouse.
Type "/vfm" in chat or use the menu entry in the interface menu (opened from the bottom left corner).