Introduction
There is so much to know about Filters and Plug-ins... I couldn't even try to cover them all in one tutorial but what I do want to do is explain what they are and what you can do with them. They all basically work in the same way and most of the artistic filters can be found in the "Filter Gallery". There isn't much that you can't easily do to an image in the Filter Gallery, from turning a color photo into a water color painting, or a chalk drawing, or even make it look like a tile mosaic. And you can add filter layers on top of filter layers, reorder them and make them visible or not visible. Some of these we will cover in other tutorials so here I'm just going to show you the basics to get you started then you can spend the next month or two creating with them. Watch the video at the end for further explanation.
Filters can also be referred to as "Plug-ins". There are a ton of popular 3rd party plug-ins out there and a lot of them are very user friendly and very powerful, I collect them like some people collect fonts (which I collect too). However, the newer versions of PS have some really great ones built in.
So lets do something fun, lets take a photo of a sunflower from my garden.

"Right Click" on the flower image and copy it. In Photoshop "CTRL+N" will open a new file. The image size is automatically set to the image on your clipboard. Click "OK" and "Paste" (CTRL+V) the image onto the page.
Now navigate to Filter > Filter Gallery and a new window will open with a list of filter options and controls. There are 6 folders with a variety of filter options in each. Start by clicking on any of them to see what they do.

This is a typical "Filter Gallery" screen. As you see here, I have added a second layer, you can add as many as you need. In this example I added a layer for Texture to give the Dry Brushed image a Canvas look.
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Artistic > Dry Brush
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Artistic > Dry Brush & Texture > Texturizer
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You can get to any of the filters from the Filter drop down menu as well as through the Filter Gallery. This is probably one of the first things a new Photoshop user will explore as they are quite self explanatory. The main thing to remember is to try them, learn what they can do and refer back to them when you need a unique effect. Keep in mind that a "filter for filter's sake" doesn't always make a good design.
Try making a duplicate layer and adding a filter to the top layer. Then use the Opacity slider to affect the top layer - this is a great way to make subtle changes to an image.

Sketch > Stamp
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Sketch > Charcoal
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Brush Strokes > Ink Outlines
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Stylize > Glowing Edges
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Many filters that you install will consist of just one file that has a .8BF or .8BI extension. They will almost always be located in the Photoshop Plug-ins folder: C:\Program Files\Adobe\Adobe Photoshop CS5\Plug-ins\Filters. So if you download a new filter, just place it the folder and Photoshop takes care of the rest. The next time you start Photoshop, the new filters will be ready to go!
Conclusion
Filters are a lot of fun and very useful for doing a variety of things, experiment with them and learn what they do, you will find them to be a very useful tool. Filters are handy, but don't forget that you can create a unique look using other tools well. Download the free 'Optional Filters' from Adobe for more effects. Also check out tutorial 3-5 for more information about using filters to sharpen images.
Reference
Here is a link to a selection of free Filters and Plug-ins. Here's another site.
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