The internet is one of those places where you can find a lot of information about anything really, but for looking up health problems you want more of a reference than Joe Schmoe's Health Blog.
SelfMD.com is a great new tool for self diagnosis online. Basically you go through a series of questions and answers and then SelfMD gives you a few options on what could be ailing you based on likeliness. The questions are basically yes or no and the diagnosis is fairly easy to read (i.e., not done in crazy doctor jargon).
Keep in mind that SelfMD is just a reference tool (a quick way to educate yourself) and if something seems not right, you should always see a doctor.