Structural Detailer
An excellent guide and reference for structural detailing matters related to steel structures and connections. The author(s) offer a very comprehensive commentary on important key considerations.
Eng-Tips
A website you've likely seen or even visited before. It is a great forum for structural engineers and other disciplines alike, with many interesting topics.
Kenchiku-Kouzou (Japanese)
If you've lived and worked in Japan as a structural engineer, then you might have stumbled across this website more than once. The author offers very clear and concise explanations of engineering principles with easy to understand illustrations. This website is also the inspiration for my website!
Steel Construction
A specific wiki related to steel construction with lots of excellent resources
Designing Buildings UK
A general wiki about the design of buildings. It covers various disciplines and although it does not cover structural matters comprehensively, it does cover a very wide range of topics.
Structural Guide
A general structural engineering website with calculations and explanations on various topics.
StructX
A general structural engineering website with calculations and useful tools.
A Builder's Engineer
A general structural engineering website with calculations and explanations on various topics.
Civil Jungle
A general structural engineering website with calculations and explanations on various topics.