Research Methods Knowledge Base This is an excellent on-line textbook of social research methods. It is well presented and covers the research process from formulating research questions to sampling, measurement, research design, analysis and writing a research paper. (It should be noted that the latest revision available at this website is for 2000 and more recent revisions are only available with payment).
BMJ series of Statistics Notes Covering a wide range of statistics topics this is an excellent series. Starting in 1994 almost all are available on-line. They are conveniently collected here on G. Dallal's web-site.
eBMJ Statistics at Square One This is a really good introduction to medical statistics from the BMJ. It is in book format which can be hard going on computer but the text doesn't tend to be too lengthy.
The Little Handbook of Statistical Practice Gets into more detailed analysis. Again it's a book, so lots of text!
Surfstat Australia Another on-line statistical book. It is useful but isn't specifically medical and gets quite numerical and mathematical (which might put some off!). It also gives the commands for Minitab which is a statistical computer package, but you can ignore them if you're not interested in it.
Statsoft Electronic Textbook this is not an introduction to statistics but it covers extensive topics, including lifetables, and so it may be useful as a reference for a tricky topic.
A New View of Statistics this textbook is not well presented, however a wide range of topics is covered and it may serve as a useful reference.
American Statistical Association Series: What is a Survey? This is an excellent series on surveys and includes a module on questionnaire design.
Trent Focus is a Primary Care Research Network in the United Kingdom. This is a link to some of their excellent research resource packs (pdf files) on a number of research methods topics, including qualitative research.
Statprimer This is an "on-line public domain biostatistics instruction manual" published by Bud. Gerstman of San Jose State University. It is produced mostly in text format in short Microsoft Word documents, but is pretty comprehensive and it is specific to the biological sciences.
Supercourse: Epidemiology, the Internet and Global Health This site is health-related and is particularly geared towards those with an interest in public health. It covers many topics other than epidemiology, which may be of interest to anyone in the health field. You need to visit the site yourself to get a feel for the breadth of topics covered.
Stat Web This site published by the University of Durham in the UK is to facilitate students in the biological sciences in learning the basics of statistics.
Pitfalls of Data Analysis Worth reading if you want to avoid them! From Clay Helberg at the Research Design and Statistics Unit at the University of Wisconsin Schools of Nursing and Medicine.