Spiral Graphics is offering two free programs that generate seamless tileable textures. If you model with card or paper, or want to print your own texture sheets to cover other materials, you must have these.
The first program is the Genetica Viewer, which has a built-in library of textures, including many of interest for structures, floors, roads, etc. Though the free viewer version of Genetica does not allow you to create new textures from scratch (a rather complicated process involving defining and connecting algorithmic units together), you can alter quite a few parameters of the inbuilt textures including the density, hue, contrast, and the random seed which varies the patterns. Your customized texture can then be output to a .jpg or .bmp file, in many different pixel resolutions, for use in drawing, word processing, or illustration software. You'll want to make sure that the file's pixel resolution, once scaled, yields around 300 dots per inch. For example, if the printed texture will have tiles that are each around two inches square, your resolution should not be less than 512X512 pixels. Finally, note that many more free textures can be downloaded one by one from their user forums. If you use BitTorrent, many of these additional textures have been put together in this torrent file.
The second program is their Wood Workshop, which works similarly to the Genetica Viewer but which focuses exclusively on wood textures, with many more customizable parameters. Clapboard and board-and-batten are only the beginning.
The primary users of these programs are those designing 3D objects for computers games or art, but the possibilities for card modeling and texture sheets are truly extraordinary.