for me it's in PS Elements, but the one used the most, and thus my personal favorite, is the Melancholytron filter. It has so many variables, a good preview screen and is totally reversible from the Edit menu.
The effects can go from subtle (for example, a slight vignette of edges) to sepia toning. It's a great filter when not overdone, and when overdone if you want to!
Noise Ninja. It can do wonders to the quality of your photographs.
I was pretty desperate to get a better camera, but now with a few minutes in noise-ninja I'm really impressed with how some of the shots actually look. Sure, a better camera will make them better still, but right now I have other things I'd rather spend my money on, and purchase the camera at a later date.