MMF2 has an "Onion Skin" option for ..

[Redsonic]

The animation editor

Syncing animations can be a pain sometimes; it'd be nice to be able to see the previous frame of an animation faded out on the background of the current frame, so it's easier to align things

Macromedia Flash has this feature as well

[Looki]

Between the list of frames and the canvas you draw in, there's a small slider. You can either view the next or the previous frame with a given transparency.

[DizzyDoo]

There is.

When editing a frame, at the bottom of the window, just above the directions box, there is this slider. Slide the slider, and you'll see a transparent onion skin of either the next or the previous frame.

EDIT: Curses Looki. I should type my posts faster.

[Redsonic]

oh my god. I feel like an idiot, because i've been using MMF2 for almost a year now

Thanks guys ^_^