Amazon, in its SEC filings, credits their Linux migration with millions of dollars in savings per quarter

"The decline in absolute dollars spent during the three months and nine months ended September 30, 2001 in comparison to the corresponding periods in the prior year primarily reflect our migration to a Linux-based technology platform that utilizes a less-costly technology infrastructure, as well as general price reductions for data and telecommunication services due to market overcapacity." [ October 30, 2001 10-Q SEC filing ]

Read more