Biology, Biochemistry, Chemistry, Earth Sciences, Biomedical Sciences, Medicine
For a full list of the different science courses on offer, see the A-Z of undergraduate courses, where you can read about each of them in detail. You might be interested in having a look at some of these resources:
NASA's Five Ways to Find a Planet tool
This radio documentary about humanity's impact on the earth
Killian's review of this book about extinction
These dispatches from the frontiers of world health, by practicing medics
These lectures about medical science from Gresham College
Articles from the New Scientist
This website about the things that scientists don't know
This inter-disciplinary science journal
Oxford University's own science research magazine
These reading suggestions from Cambridge University
These demonstration lectures from Cambridge Chemistry
Lectures on Biology from the University of Oxford (though remember most of these will be aimed at university students)