Here is a (huge) compilation of the best free Photoshop brushes out there. They are the ones I use, and the ones it took me a very long time to find over the web.
I've selected the best of them, and along with a preview and a description, I've made a zip file that contains the whole compilation.
For all brushes hosted at brushes.500ml.org you'll have to navigate to the brushes, since they don't allow direct linking...
If you enjoy this set of brushes consider blogging about it ;)

Take into account that this is a huge list of brushes, and it takes a long time to download. Also, the mirrors might only be available for a limited time, so if you find them to be broken please notify me
And here is the list (Alphabetically ordered)

Art nouveau

This beautiful set features 8 brushes with patterns and designs following art nouveau techniques and style.
Visit the brush homepage
Art Nouveau brush set

Botanical flowers

This is a very nice set of brushes, specially for "natural" websites or just as footer decoration.
Visit the brush homepage
Botanical flowers

Bugs and Angels

This is indeed a very strange combination of brushes to put in a set, but both of them are very beautiful and make a good "innovative" design element.
Visit the brush homepage
Bugs and angels

Elegance

One of the best web design brush sets I have ever found. This provides you with some very elegant lines and circles. Worth checking out ;)
(I couln't find the author's website, so if you want the set, download the pack)
Elegance

Feathers

Strange brush set, filled with feathers, but it makes a beautiful texture I guess.
Visit the brush home page (Scroll down a bit, by Sheishido)
Feathers

Floral swirl

A very beautiful brush set, largely used in modern web designs (And in this blog's design too)
Visit the brush homepage
Floral Swirl

Foliage

Another brush set of plant elements. This time it is packed with tree branches, leaves, bushes... Great for some "wild life" designs:
I couldn't find this brush homepage :S
Foliage

Fractals

A nice set of fractal images, these are great to put in backgrounds for scientific websites or pics...
I didn't find the original brush set website, but here at getbrushes.com you can find tons of fractal brushes.
Fractals

Frames

This are good sets (I've put here two sets of frames) to enclose your images in.
Frames 1
Frames 2
I don't remember where I found them, but just search for frame brush sets and you'll find a lot more.

Garden of Decay

One of the most beautiful sets here, nice textures and images. It comes with some handwritten text too.
You can find it here, along with a lot more brushes by Annika
Garden of Decay

Good & Evil

A nice set, featuring some trees, skies, and other textures. Great for image edition, and adding header decoration too.
Brush set website (And also in Annika's website)
Good & Evil

Grunge flowers

Probably the best brush set exposed here. I used it a lot for my backgrounds in this blog's design.
Grunge flowers

Grunge texture

A set of textures to use in "rough" designs probably
Grunge texture

Hair

I really don't know where or why you would use this, but it is a good set of high quality "hairs" :S
Hair

Illusionist

Very strange set of cut & inversed faces. I really don't know how to describe this set. It is very cool though, so check it out.
By Annika Von Holdt
Illusionist

Paint

This is a very useful brush set, it is also the one I used for the header background, those paint strokes. It has both strokes and splatters...
Paint

Patterns

Some patterns for backgrounds or buttons, or whatever you want them for.
Patterns

Splatter

Another set similar to the paint set, but this time focused on Paint splatters. If you want to add splatters to your designs, this is your set!
Paint splatter

Valentine

A very romantic set, with hearts, roses, hand-written text... You'll love it ;)
Valentine

Victorian people

This is probably a very strange brush set you might think you'll never have to use, but once you have it, you'll use it. Guaranteed. It features full-body pictures of Victorian people.
Victorian people

Important notes & download

Please do not hotlink the images nor the zip file with the compilation, instead link to this article please.
I couldn't find most of the brush sets websites because over the years I have renamed most brush set names, so if you are the owner or creator of them and I have not placed a link to you please comment it below.
Also, if you think a link was misplaced do comment.

Download the compilation

Here you can download all of the brushes before. They come in a zip file, ready to send to photoshop.

You don't know how to import them in photoshop?
Simply go to the install location of photoshop (Normally in C:\Program files\Adobe\Photoshop). Open it up and search for a folder called "Prestablished settings", once there go to "Brushes" and paste the content of the zip file.
If you had Photoshop already opened you'll have to close it and re-open it to use your new brushes.

Enjoy :)