I have seen many people asking for information that can be had on the net. I think a post with sites that offer good information is needed.
Here is a list of the sites I have come across...
I think this is one of the best sites for repairs and adjustments.
Sometimes a video is the best teacher.
Need specs on a particular bike? This site has info on numerous brands and goes from 1993 to 2010.
If you like building your own wheels, this site has spoke measurements needed for many different hubs.
This site describes how to mix and match different types and makes of rear gear mechanism, shift lever and sprocket cassette or freewheel.
Please add your favorite sites.