In reply to dan tipper:
The ones I've looked at in the past are:
Action Challenge (good if you want to do Kilimanjaro - which is great for gaining experience in treating altitude related disease).
Charity Challenge.
Raleigh International.
British Exploring Society.
Exile Medics.
None of these 'employs' you exactly, but will generally take you as a 'volunteer'. You'll likely have to pay a little for indemnity insurance, or some admin fees, or pay for your travel, but you get good experience and may get to go to some cool places.
The only one I've worked for is Action Challenge, and they are good but only seem to offer Kilimanjaro until you've worked for them for a while. They also do UK events which I think pay a small wage.