For seamless textures I only use Paint Shop Pro 9, it's got just the right level of simplicity to do this stuff quickly without needing automated filters.
On a solid image, say a wall photograph I would cut out a square, trying to think about what you grab inside it - like if it's bricks then try to get the scale right so it repeats.
Then I select half of the image, splitting it horizontally so you get a top and bottom half - then I flip the selection, invert it to select the other half, then flip that as well. So the part of the texture that is fine, the middle, has been switched to the top and bottom edges and you know that they are seamless. Just a matter of using the clone brush to get rid of that new seam in the middle. Try and get rid of this while keeping the texture neat.
Now once the middle seam has been clone-blended in, you could scroll that image vertically and it will repeat. It's then a case of doing the same thing but splitting vertically this time, and working with a left and right half, mirroring instead of flipping.
Once the vertical split has been done you should have a fairly tileable texture. I tend to split vertically then horizontally again but at 3/4 and 1/4 sections so that the image is offset and any problems will end up in the middle, not at the sides. Once you've done this the middle of the texture can be adjusted as long as you stay away from the sides. For instance a slightly darker patch will ruin your seamless texture as it becomes an obvious repeat when you see it a hundred times on the screen. So clone out any nasty bits you think might cause problems, fix the colour, get it as neat as you can get it.
Once you know these techniques there's no real need to use filters - in fact this technique lets you adapt to the shape more neatly and with practice you will get great results doing this. Irregular shaped bricks for instance can be cloned entirely and that's a great way to keep your texture looking believable - it does take time but the results tend to be worth it.
I wrote a seamless texture art package years ago, but the one thing I loved the most from it was that scroll feature - you could just right click and drag the texture about the place. It would be great to see that feature in art packages, would make this sort of work so much simpler I think
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