Zalman recently released the CNPS6500B-series heatsinks. Designed for the P4-series of processors, these mammoth heatsinks weigh in at a whopping 898g. (over 6 times heavier than the earlier CNPS3000-series.
There’s even a warning on the 6500 page telling you that this heatsink exceeds Intel’s own 450g maximum weight recommendation for heatsinks, and to proceed at your own risk. Yikes.