X-ray astronomy

SRON Netherlands Institute for Space Research
http://www.sron.nl/

SPEX X-ray Spectral Fitting Package (Forum)
http://www.sron.nl/phpbb

XMM-Newton on the ESA website
http://www.esa.int/science/xmmnewton

XMM-Newton public image gallery
http://xmm.vilspa.esa.es/external/xmm_science/gallery/public/

Popular astronomy

Public Observatory "Philippus Lansbergen"
http://www.lansbergen.net/

Jelle de Plaa (Popular Astronomy and photography site)
http://www.deplaa.net/