Here’s a list of sites I find very useful and informative… and fun. 🙂
http://www.2bike.rs/ – Serbian cycling portal with 2nd hand parts and bicycles yellow pages (with advanced search options) and a very good and informative cycling forum.
http://www.bikeforums.net/ – USA cycling forum. Some very good cycling experts willing to share their knowledge and experience. Also lots of first hand experiences with a vast range of parts, equipment etc. Very informative. Sheldon Brown (R.I.P.) was a member in his time.
http://www.ravingbikefiend.com/ – A blog by a very good mechanic (and an extraordinary man). Especially interesting is the part about winter cycling, making DIY studded tyres, as well as winter bicycle maintenance.
http://www.sheldonbrown.com/ – Site of the deceised legend Sheldon Brown. It should be protected as a treasure of the civilization. If there is a cycling mechanics bible, this is it.
http://www.gear-calculator.com/ – Bicycle gear calculator – a very useful tool.