Club and Friends (in alphabetic order)

AAR – Amateur Astronomers of the Rheingau e.V.
Gordian Kley – Shows how to do decent astrophotography even from the heart of the Rhine-Main-area
Thomas Klotzbücher – co-founder of the Rhine-Main-Astromeeting
Ben Nagorsen – colourful observing reports, even several quite old ones with me

Useful links

Astronomy Picture of the Day – Every day a new Astronomy-related picture
Starship Asterisk, the discussionsite for the Astronomy Picture of the Day is even more interesting than the original APOD-site
Sky and Telescope
IAU-MPC – Minor Planet Center
AXA – Bruce Gary's exoplanets-website with a very thorough handbook for aspiring observers of exoplanet transits
ETD – Exoplanet Transit Database
BAV – Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V
Aurora-activity – aurora information
Cloudy Nights – american astroforum
WebAstro Forum – french astroforum
Astronomie.de – german astroforum
Astrotreff.de – german astroforum

Weather

EUMETSAT – Satellite-images from Meteosat
SAT24 – up-to-date Meteosat-satellite images and animations for Germany (visual and IR) – every 15 minutes
Meteoblue – relatively reliable weather predictions
Wetter Online – relatively reliable weather predictions

Expert Deep Sky Imagers (in alphabetic order)

Markus Blauensteiner
Chilean Advanced Robotic Telescope 32" – Great collaboration, cool telescope and a perfect site
Bart Delsaert – Excellent gear and very good location. Aesthetical pictures.
Christian Dupriez – Very busy imager from Lille
Bernhard Hubl
Rick Johnson's 1600 Image Collection and Cloudy Nights postings
Fabian Neyer
Andreas Roerig
Derek Santiago (schmeah)
Johannes Schedler
Elke Schulz – outstanding images
Joel Short (Buckeyestargazer)
Jim Thommes