If you find yourself using photoshop a lot and are looking for more ways to further enhance your art, then looking into commercial plugins might actually be worth the price. There's many many free resources out there, many are very good too. But there's an even bigger selection and more hidden power to be explored once you're ready to pay a little. If you're using photoshop to make money, chances are that it will be worth it.
There's a few plugins I myself have bought that I think are worth every penny and i'm going to shortly try to explain why. I would advice you to download a trial version before paying anything though, just to see if it works for you.
One of the best plugins/stand alone programs money can buy. Anything you need will most likely be found in here. Their newest version now supports both layering and masks, two things that are extremely useful when applying your effects. You will have an advantage if you buy their stand alone program over their plugin so I advice you to look into the differences before purchasing.
The most amazing thing with this program is that it gives you so much control. You can apply effects that are seemingly subtle and then be amazed at the difference in your photograph. There's also some amazing special effects to be found here, it actually has an entire section devoted to it. Their lighting efects are also the best on the market, and i'm a big fan of their film lab. The most fun thing is to experiment and combine different effects, I have turned regular photos into artworks just using this plugin alone. I strongly advice you to take it for a test spin, I don't think you will be disappointed.
Nik Color Efex Pro
Another amazing plugin worth every penny. It has so many different filters to choose from, and it ranges from photo realistic to surreal. There's few of the effects that I never use, and more often than not I open the plugin without having any idea what effects to apply. That's the fun thing about it, that you never know how one effect will look on a picture. You easily and quickly switch between effects and you can always customize it to your liking. It's so easy and fun to use, and it can lift your photography to new heights. It has a darken/lighten center that add focus and depth to any photograph and I use it almost every time I open photoshop. I also use their dynamic skin softener often with my portraits. They also have highly advanced effects such as "sunshine" that can actually create a realistic sunny day from scratch. Their "tonal contrast" is incredible when you want to enhance details and lift your photograph, and their "old photo" take your photography back in time and simulate that old school feeling n a beautiful way. They also have both color and black and white infrared effects that I use a lot I think I add atleast one of Nik's effects on every photograph I take into photoshop, that's how useful it is.
Oneone software's PhotoTools
This plugin works a lot like photoshop actions but in a much more flexible and clever way. You can layer effects upon each other and customize your photo according to taste. You can preview the effects and choose from a long line of different effects, such as color, sharpen, frame and borders, special effects and even HDR simulation. You simply click a button and the action will do all the work for you. I find myself using this plugin a lot when in the mood to experiment, it's a lot of fun trying different effects and layer them on top of each other. You can easily remove effects you don't like and control their strength.
These are only three plugins that I love, there's so many more out there. Try some different ones using their trial, it will often give you an entire month to try it out. Try it on different photos and different art, dig into the effects and look at all its pros and cons. You also have to ask yourself if these effects are worth the money or if it's effects you can create yourself in photoshop and do you have the time and energy to do so?
I would also suggest you to go for a plugin/software that mostly fit YOUR needs, and have the kind of effects you could see yourself using. Their product must be able to enhance your product which is your photography.
Best of luck to you!