This brush will make your strokes look like they were drawn with a pencil. Thin were the stroke begins, wider in the middle, and back to thin were the stroke ends.
Each size comes in two variations - one with the stroke getting wider to the left and one with the stroke getting wider to the right.
If you need other sizes just copy one of the sizes and modify it.
Put the .ai-file in \Adobe Illustrator\Presets\Brushes and restart Illustrator.
If you use this brush feel free to link to your deviation so I can see what you can do with it.
Edit: I have included another version of the brush for those running older versions of Adobe Illustrator. Thanx ~tracksix for the info on how to convert it.
Use Lineart_Brush_by_jimro.ai in Adobe Illustrator CS2
Use Lineart_Brush_by_jimro_AIv10.ai in Adobe Illustrator 10 or CS