Just one thing to check: you don't happen to have a file called 
"digest.rb" lying around in the current directory?

It's tempting when writing a test program to exercise the digest library 
to call it "digest.rb". But then "require 'digest'" will fail because it 
will load the wrong file :-)

One way to diagnose this is to find digest.so on your system (e.g. 
"locate digest.so") and then require it with a full path: e.g.

irb(main):001:0> require '/usr/lib/ruby/1.8/x86_64-linux/digest.so'
=> true
irb(main):002:0> Digest::Base
=> Digest::Base

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