I saw this while browsing the Merb source code, and despite the bad variable names thought it was pretty nice:

module Enumerable
def injecting(s)
inject(s) do |k, i|
yield(k, i); k
end
end
end

Use like:

\[‘o’, ‘hai’, ‘rly’, ‘srsly’\].injecting({}) {\|accumulator, value\|
accumulator\[value\] = value.size}  
\# => {“srsly”=>5, “o”=>1, “hai”=>3, “rly”=>3}

Simple pleasures. I find it increasingly hard to live without things like this, Object#returning, and Symbol#to_proc as I start to use them. After all…