Painter has a nice feature that can be used to create freehand seamless web tiles. Any brush can be used to create these tiles. [Painter does need to be upgraded to 6.03 or better to use the new grad and pattern ink brushes. Without the upgrade, these brushes do not wrap around a drawing surface.] 
It's a fairly simple process to create a drawing web tile. As a starting point, create a new blank file, 150x150 pixels. Go to the Select Menu and pick All (or use CTRL-a). This obviously selects the whole image and the "marching ants" surround the file.
Go to the Art Materials Palette, the Pattern section and click on the Pattern section menu (the triangle at the far right of the section). Click on the Define Pattern line. This will turn the blank square into a seamless tile. Draw on it and the line will go from one edge to the other.
There's a lot of different techniques that can be used.  A brush can be used on its own in a freehand fashion. This one is a simple example. A tan Felt Pen Marker was scribbled over a light ochre background. A webpage background using this tile is here.
This pattern uses a rainbow gradient mixed with the Distorto variant of the Liquid brushes. Using a diagonal stroke helps to get rid of the square appearance that a lot of homemade seamless backgrounds have. While there is clearly a repeated pattern, there's no seam to give away what the original image looked like. A webpage with this tile is here.
This pattern has a similar technique to the rainbow one. A light blue Digital Airbrush was stroked over a blue background. The Turbulence Liquid brush variant blended the colors together to present a smoke appearance. The webpage sample is the background on this page.
This nebula look was gotten by painting with the Glow variant of the F/X brushes. By painting with large circular strokes that extended outside of the 150x150 pixel frame, it was possible to present an illusion that a much larger tile was used. A sample webpage is here. One note is that this, and a number of other F/X brushes, do not create seamless patterns. They appear to as brush strokes go from one edge of the image to another, but seams remain when tiled. The Smear brush variant of the Liquid brushes helped blend the edges. 

These are only a few examples of what can be done using drawing techniques to create seamless tiles.  More examples are below.

Blue Pencil

Bristle Spray (Impasto)

Fiber (Impasto)

Felt Pen

Leaky Pen

Grad Pen