You could use water cooling. From what I've heard it's booth
really quiet, and more efficient (keeping the CPU at a lower temperature).
The bad thing is the price I guess.... and the fact that it needs some space...

For some low-energy processors (like those VIA builds into some micro-ATX motherboards)
you could even get away with passive cooling, but that isn't an alternative for most,
since those processors are quite slow...

/Irkasmork