IEEE Micro: Web Search for a Planet: The Google Cluster Architecture. [104K PDF]
The gritty details of how Google's 15,000 commodity-class PCs with fault-tolerant software achieve superior performance at a fraction of the cost of a system built from fewer, but more expensive, high-end servers.
Nothing all that new, but this arcticle goes into greater technical detail than what I've seen before.
[Google Weblog]