[imurray]

Obvious choices are big firms with machine learning research groups: Microsoft Research, Yahoo Research, (Google, but I've had a poorer personal experience getting people from there to tell me what they're doing)

You could search for funds and consultancies using machine learning. They are out there.

Check out the sponsors of major machine learning conferences (e.g. ICML, NIPS).

[pgbovine]

research labs typically only hire Ph.D. students as interns, not master's students. an alternative choice might be start-ups who have a need for machine learning (e.g., collaborative filtering for e-commerce). i can't imagine why they would turn down a competent summer intern who didn't ask for equity. perhaps ask around for YC companies who are hiring interns.

[ephermata]

This is true, but there are plenty of other teams within Microsoft (or Yahoo or Google) that do applied machine learning. You probably won't end up with a publication in a peer-reviewed conference, but you will learn about the state of the art. Of course if the goal is improving your CV for application to a PhD program, that may not help much.

[imurray]

http://research.microsoft.com/en-us/jobs/intern/faq.aspx

The majority of our interns are PhD candidates, however, we also accept some master’s students who demonstrated a serious interest in research.

Point taken. The poster needs to decide how academic research focussed they are.