There are millions of free Photoshop plugins out there, but few are actually useful. These are a few I have found that I really like.
GML Growcut: a very easy way to make selections (the hardest part of Photoshop.) The free version can be found here: http://www.foto-freeware.de/gml-growcut.php
Fast Fourier Transform: there are many tricks you can do with a Fourier Transform. The key one is removing a regular texture from your image (for example, if it's scanned from a magazine.) You'll need to read a tutorial before you can do anything with this.
Flaming Pear: the ones I use the most are Solidify A, which fills in (in a blurry way) an area you delete from the image, Ghost which turns white to transparent, and RGB->HSL, for when you want to mess with the Hue or Saturation channels.
Fantastic Machines Paint Engine: The old Beta works fine and is really good for painterly effects. It takes some experimenting with to get the best results.
SAT deblock: JPEG artifact removal. It really works better than other methods of fixing overcompressed JPEG's. Most blur away the artifacts and leavy blurry patches, but the shape adaptive algorithm only blurs from directions that don't cross edges. http://www.cs.tut.fi/~foi/SA-DCT/#ref_software
Insta-Sketch Beta: For turning photos into pencil-sketches. Works best on a plain background. He has a few other NPR actions here you could try out, too. http://homepages.slingshot.co.nz/~mikefinn/action.html
If you want to buy something, I'd recommend Smart Fill from Alien Skin, Buzz Pro from Fo2Pix (defunct?), or India Ink from Flaming Pear.