http://tools.wmflabs.org/robots.txt:
User-agent: *
Disallow: /
Tools are obscure and hard to find enough without forbidding search engines to do their job as they do on Toolserver...
(In reply to bug 59118 comment 5)
Set up robots.txt as a temporary measure to:
User-agent: * Disallow: /
Version: unspecified
Severity: normal
URL: http://tools.wmflabs.org/robots.txt
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=61133