If you're using Photoshop CS4 or 3, then I wouldn't really use any other program, unless you want to go with Photoshop Elements. If you want alternatives, you can use Paintshop Pro or Lightroom 2 (Adobe product like CS). And if you run NIX, you can use GIMP, however it's available for other OS.

Check out Photoshop | Open Source Alternative - osalt.com, they've got some good information.

You can always download plugins to run along with Photoshop, like Eyecandy, which is a part of Alien Skin Software (Alien Skin Software, LLC). Check out http://www.thepluginsite.com/resources/freeps.htm to find plugins that are free that you can run along with Photoshop.