I've been looking around for (post-grad) internships for the last few weeks. The most successful tactic so far has been to just email and ask directly. Make sure to link to some existing projects that you can show off so they know you won't just be dead weight. Show that you have actually researched the company and are interested in what they do. I've had a few interviews already with this sort of thing:

  > Person X, I saw your job posting on HN and wonder if you also accept interns?
  >
  > I have professional experience with python, erlang and ocaml and have
  > previously worked in a startup environment. I have
  > little practical experience in machine learning or data mining but I
  > do have some classroom learning as well as a solid background in math.
  >
  > I have a resume and portfolio online (
  > http://scattered-thoughts.net/all?jid=2&tag=about ). A few of the
  > smaller projects in there are vaguely relevant to Company X - a personal
  > job hunting agent (
  > http://scattered-thoughts.net/one/1274/311469/330868 ) and an email
  > chat bot for 419 baiting (
  > http://scattered-thoughts.net/one/1274/311469/329424 ).
  >
  > I am hoping to start a phd in September so I cannot accept permanent
  > positions at the moment. I am not a US citizen but I am eligible for a
  > J1 visa and able to move to SF without financial assistance.
  >
  > Thanks
  >
  > Jamie

Edit: Fixed the formatting