Lots of filters do cool/ 3D/ illustration/ etc. stuff, but I think color filters are the real deal.
Try Nik's Color Efex 2.0 filters... you can buy them as a complete set or in several collections... and you can "brush" the parts you want on a layer mask and even gaussian blur it to naturalize it!

(Although brush mode makes a composition of the image, instead of just a layer)
If you want special effects, try Alien Skin' Eye Candy 5/ Xenofex 2 filter sets.
Incredibly powerful. But for image tweaking they do little good.