The following websites have interesting and useful information about bastioned fortification, in one way or another. If you find any other websites that you think should be added to this list, please let me know on firstname.lastname@example.org
Wars of Louis Quatorze Blog - An interesting blog about the wars of Louis XIV, which sometimes has some good posts on fortresses and sieges.
Staats-Spaanse Linies (States-Spanish Lines) - Very informative site on the fortified lines and towns along the northern border of Flanders that were built during the 80 Years War.
Muuka.com - Excellent site with a section on various fortresses in Finland.
Nasjonale Festningsverk - Site about the fortresses of Norway, unfortunately only in Norwegian.
Suomenlinna - An island fortress guarding Helsinki. Built in the 18th century, Suomenlinna was a large military base, containing naval dockyards and barracks.
SpanishSuccession.nl - A very good detailed site on the War of the Spanish Succession, which saw various works of Vauban and Coehorn tested in tough sieges.
Franck Lechenet - A collection of impressive aerial views of Vauban fortifications throughout France.
Northern Fortress - An excellent site on the fortresses of northern Russia and Scandinavia.
Association Vauban - Some good information on Vauban and his contempories here.
Fortifs.org - Fortifs blog.
The Palmerston Forts Society - A group that works to preserve the Palmerston Forts (south coast of the UK).
The Catalonian Fortresses - A site dedicated to the fortresses of Catalonia (eastern Pyrenees) - Worth it for the pictures even if you can't speak any of the languages.
Vestingsteden - A very informative site on fortified towns in the Netherlands.