General modelling sites:
Aircraft Resource Center - Relatively new magazine, with walkarounds and kit reviews.
Hyperscale - Very large modelling site, an almost daily stop for me.
Internet Modeller - Monthly modelling magazine. Very good.
Modelling Madness - updates frequently, several built up and in box reviews.
PC Modeler - Doesn't update as frequently, but worth a look.
Japanese Aircraft site - just what the name implies. Good info.
Luft '46 - site dedicated to some of the (very interesting) "What If" Luftwaffe projects
rec.models.scale - Good modelling tips, news, and advice - just ask.
alt.binaries.models.scale - Occasionally shows model pictures, since it's a binary group, it does get other spam.
alt.binaries.pictures.military - Pictures of military hardware and uniforms.
Accurate Miniatures - one of the best kit makers around
Aeromaster - great paints and decals
Aztek - Some of the best airbrushes around.
Dragon/DML - Highly detailed kits
Hasegawa - Hasegawas home, in japanese
ICM - New, but very good
Revell-Monogram - One of the older kit manufacturers, but very good
Testors - Great paints.
Tamiya - High quality Japanese kit manufacturer
Verlinden - Resin updates and figures.
Catalogs and online hobby shops:
Airline Hobby Supply
Roll Models - fast service, decent prices
HobbyLink Japan - Want a Tamiya, Hasegawa, Nichimo kit? You'll probably get it here cheapest.
Hobbyweb - big online store, good service.
Squadron Mailorder - the paper catalog's better, but you get good service here.
Meteor Productions - manufacturer/distributor of Blackmagic masks, Cutting Edge decals, etc.
Marco Polo Imports - distributor of Hasegawa, Tamiya, etc.