The best way for me is doing it by hand. Is use only clonetool

They are kinda old school IMO. The grass texture is blurry and makes the original worse, not better.
The brick texture is without highpass better, too.
IMO, the way that I've seen with HL 2, is the way to go.
A texture that obviously repeats but ranges at least 2 meters or the hight of a floor, so the leveldesigner can easely gives it variability and hide the repetitions by shadows, lights and edges and decals.

As soon as you can mix such a texture with a shadow map you win.

I used it in the following screens with terrain as detailmap:





If I'd followed the first of the gimp tutorials, i wouldn't achieved this textures. (Maybe, you disagree and dislike my examples. Then please tell me and say why.)

Try Genetica Viewer from Spiral Graphics. You can get a free version. It comes with tons of seamless textures that you can tweak out and edit. You can also make your own.

But I prefer manually making them. I take real photos of surfaces/textures with my camera, and make them seamless in photoshop. Then couple that with all the materials/shader, and I have some truly photo-realistic textures.

In fact I'd not consider using any of those "auto-seamless-makers" because imo they all suck. Whenever I found such a functionality and tried it out the results were the same: Usually it's some kind of cheap algorithm / effect that blurs the seams together. The results are okayish for things like grass or like lots of little stones but when there is a pattern that has a really periodic appearance or that in some other ways has a certain pattern all those techniques fail badly as you really see how the software blurred the edges or copied some seam parts around...

Might be worth a try though if you need a result that's not fantastic but has to be done quick. So if there is some time I'd always prefer a "handjob"...

Enjoy your meal
Toast

You don't wanna read my guess: you never tried the demo of the tool I mentioned.

I have a library of self-made photo textures for "Running Food"/"World of Dice" of 100 made with photoshop (I didn't know of STG from the beginning) and at least 200 self-made textures made seamless with the Generator. Using the Seamless Texture Generator doesn't prevent you from thinking about the best frame and the best perspective and from testing whether the texture looks appropriate, but it needs only 5 steps where you otherwise need at least 30.

Well which one do you actually mean? The seamless texture maker by Genetica or something else?

Enjoy your meal
Toast

Disclaimer:
Your hands won´t going mouldy nor will your PC explode if you combine both methods.

Let the software do the dirty work and only if it doesn´t look right at some points,
use the clone tool to fix the mess.
For me STG gives the best results.

try Luxology Imagesynth 2, it is a Photoshop plugin and a standalone program that is pretty nice.
Costs a 100 bucks, but its worth checking out.

I know how to make good seamless textures, and I know how much time it costs, and I know how much time I saved with Seamless Texture Generator.

Yes it resumes pretty well the situation
I use only open source and free tools, so Photoshop is not an option for my case.