Not so subtle. These tileable wood patterns are very useful.
Source Elemis
Like the name suggests, this background image consists of a pattern of dark bricks. It may be an option for you, if you are looking for something that looks like a brick wall for use as a background on web pages. It's not a masterpiece, but looks pretty nice when is tiled.
Source V. Hartikainen
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
A free seamless background image with a texture of dark red "canvas". It should look very nice on web sites.
Source V. Hartikainen
More tactile goodness. This time in the form of some rough cloth.
Source Bartosz Kaszubowski
Formed by distorting a JPG from PublicDomainPictures
Source Firkin
High detail stone wall with minor cracks and specks.
Source Projecteightyfive
The image is a seamless pattern of a fishnet.
Source Yamachem
Here's a dark background pattern that contains a steel grid pattern as a texture. Use it as a website background or for other purposes. It's free!
Source V. Hartikainen
A pattern drawn in Paint.net and vectorized in Vector Magic.
Source Firkin
So tiny, just 7 by 7 pixels – but still so sexy. Ah yes.
Source Dmitriy Prodchenko
A seamless pattern created from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin