Carson Farmer has written some new classification functions for QGIS. QGIS 1.5 only has “Equal Interval” and “Quantiles” classification algorithms implemented. Carson added the following algorithms to trunk:

• Jenks Optimisation (or Natural Breaks)
• Standard Deviation
• R’s “Pretty” algorithm

QGIS classification algorithms for layer symbology in current QGIS trunk version

Read more on his blog: “Playing around with classification algorithms: Python and QGIS” and “Adding a bit of class(ification) to QGIS…”