Apple Xserve

Verdict: The Xserve is far from cheap, in fact it rates among the most expensive 1U rack servers on the market when compared with its Windows-based x86 counterparts. However, few servers on the market at any price can offer the same combination of build quality, performance, backward hardware compatibility and ease of deployment.
http://www.itpro.co.uk/reviews/115924/apple-xserve.html