This week Marco Hugentobler (http://sourcepole.ch) committed a nice new feature to QGIS master branch: Line pattern fills. The default selection of hatches that can be used to fill polygons in QGIS is quite limited. Of course you can always use SVG symbols to extend that, but Marco has added another option. With the line pattern fill symbol style, you can create multiply symbol layers of pattern fills with different rotations, allowing you to create a wide range of fill styles. Rather than try to describe this, its easier to demonstrate with a few images.
Of course these new layer styles can be combined with existing layer styles (SVG Fill, Simple Fill etc.) to extend the already huge range of possibilities for symbolization in QGIS. Thanks Marco for another great feature!