A very personal selection...
R/C Soaring: Mike Shellim's splendid magazine. Gliders (slope), some electric powered models, ECO Piccolo, Multiplex corner, technical stuff, lots of good links.
The Future is Electric: Ken Myers' Electric Flyers Only website with monthly Ampeer newsletter. All about electric flight, articles on several topics, model and equipment reviews, lots of useful technical information and data sheets.
E-Zone Magazine: All about electric flight, columns on several topics, model and equipment reviews, technical information, useful FAQ.
LiftZone Magazine: All about sailplanes, columns on several topics, model and equipment reviews, technical information, useful FAQ.
Envision Design: Blaine K. Beron-Rawdon. Technical white papers, model design (spreadsheets 'Plane Geometry'), design data for comparison ('Planes Database'), servo linkage design (spreadsheets 'Linkage Design'). Look on R/C Soaring too.
HQ-Modellflug: Dr. Helmut Quabeck. Models, HQ airfoils, THE (German) book on model aerodynamics and design and the (spreadsheet) program FMFM.
MH-AeroTools: Martin Hepperle. Aerodynamics, airfoils, programs for design and analysis of airfoils and propellers, propeller scanner and more.
UIUC Applied Aerodynamics Group: Michael S. Selig. Aerodynamics at low Reynolds numbers, propellers, airfoil database, UIUC Low-Speed Airfoil Tests (LSAT) of the UIUC Applied Aerodynamics Group.
XFOIL: Mark Drela. The famous airfoil design and analysis tool.
aerodesign: Hartmut Siegmann. Professional information on aerodynamics and model design, airfoils, sample projects, models etc.
Advanced Topics in Aerodynamics: A. Filippone. Many clear and vivid explanations, especially on low speed aerodynamics.
Pteranodon: Fred Ludwig (already passed away). The flying saurian Pteranodon and the stork as well flying tailless models, moreover a model airship: simply great!
fkrnts2: Johannes W. Leinauer (already passed away). 'rc - bionic thermal glider' and more very interesting models as well as ideas on model flying (soaring). Good links.
Geier-Segelflug: Johannes Huser. Serious and obviously successful project to design model gliders in concept and detail taking examples and suggestions from nature (birds).
Bionics at Berlin Technical University: Giannino Patone and Werner Müller. Research report: Biomechanics of birds feathers and their bionic realization.
The Albatross Project: Wake Forest University. Distinct explanation and animation of dynamic soaring.
RC Helicopter Web: John Vugts. Good information for r/c helicopter beginners. Training models (parameters) and demo flights for REFLEX simulator, videos and pictures (including crash pictures), interesting links.
Fly Heli: Kurt Moraw. Lots of good (even if old) information for beginners and experts, regarding heli models, simulators and electronics. Videos and pictures, interesting links.
Practical Theories: Colin Mill (creator of the very realistic CSM simulator). Very informative articles on model helicopter technology, setup, and flying. Not really new (1995 - 1997), but still completely valid.
REFLEX XTR²: by Stefan Kunde. Top sim for helicopters and 3D/pattern aircraft, good for other aircraft and even slope and thermal soaring. Webshop.
crrcsim: Charles River Radio Controllers Simulator. Aerodynamics by Mark Drela. Perfect thermalling trainer, several models, nice sceneries. Best with R/C Joystick, but also usable with mouse.
sss: Slope Soaring Simulator by Danny Chapman. Really great for slope soaring, but thermalling as well, even slope combat with robot models. Very interesting terrain, sound. Best with R/C Joystick, but also usable with mouse.
PicaSim: R/C flight simulator by Danny Chapman. Really great for slope soaring, but thermalling as well. Very interesting terrain, sound. Best with R/C Joystick, but also usable with mouse.
Flight Gear: Open Source, modular, several physics modules, many aircraft, very realistic sceneries. Very interesting. Major drawback: no models, only 'real' aircraft.
R/C Simulators: German site on all model flying simulators. Many models and sceneries for download. Several forums.
Aerobatics - Top Secret?: The neXT R/C heli flight simulator, but also many heli models and very good variants of the Katana for REFLEX XTR for download, and aerobatics videos, all by Klaus Eiperle.
www.Szenerien.de: Very nice sceneries for REFLEX and other major simulators, instructions, workshop and much more, by Harald Bendschneider. Look at his other website www.Sceneries.jimdo.com (workshop) as well!
Sceneries for R/C Flight Simulators and Tools: Very nice up-to-date sceneries for REFLEX and other major simulators, SDU tool, RSK workshop, by Paul Dürr.