Rollpie Wood Grain is a free grain generator that runs in your browser with no install. Rings, knots, and pores are all computed, so the pattern never blurs at any export size.
In the color panel, the six presets from oak to dark brown swap the light and dark colors. You can then change either one to any color you like in its own field. The shape of the rings stays exactly as it was.
In the grain panel you set the ring density, warp strength, wave scale, streak roughness, knot count and size, angle, and contrast. Drawing a new seed replaces the knots and the warp all at once, and the same seed always gives the same grain.
In the export panel you give a width and a height and save a PNG. The preview on screen is not scaled up: the image is drawn again at the size you asked for, so even large exports stay sharp. It is ready to use as a background tile or as a texture on a 3D model.