The following sites are only a very limited selection but in my opinion they represent the currently best and most interesting ones on the WWW. They all have a large amount of information of almost every volcano and cover many aspects of volcanology. Links to virtually all other relevant sites of interest can easily be found there.

• How volcanoes work by Dr. Vic Camp, Department of Geological Sciences, San Diego State University is an excellent, well-designed and rich educational resource on nearly all aspects of volcanology. Award winning and of text-book quality, it is easy to navigate and not overloaded, suitable both for students and the interested layman. Contains excellent photographic illustrations of volcanic features and a well compiled list of links. Highly recommended.
• SOL - Stromboli On Line An amazing site, originally dedicated to Stromboli and its seismic activity monitored by the R. Carniel and colleagues from Udine university, has now grown into a rich resource of information, photos and videoclips on many other active volcanoes as well. Most of its material, of course, is still on Etna and Stromboli. Created and maintained by Jürg Alean and Roberto Carniel, this site comes furthermore in 3 languages (English, German and Italian). Enjoy some of the very best volcano photos available, mostly by J. Alean and M. Fulle (a friend and my 'teacher' in volcano photography) !! 
• Smithsonian Institution - Global Volcanism Project (GVP). THE source of scientific and up-to date information of holocene volcanoes worldwide. Gathers reports from current eruptions and publishes the Bulletin of the Global Volcanism Network (GVNB) (online available). Very detailed information and a comprehensive archive of volcanic activity of in parts since 1968.
• U.S.Geological Survey, Volcanoes Hazards Program: an incredibly rich source of information about volcanoes (mainly in the U.S.), volcanic hazards and monitoring, case studies, and a loads of educational and open-file scientific material.
• IAVCEI (International Association of Volcanology and Chemistry of the Earth's Interior). Reports on current research projects, meetings, conferences and workshops, links and addresses in volcanology.
• L'Association Volcanologique Européenne (LAVE), maintained by Dominique Decobecq, is a large volcanic site in French with lots of information, pictures, films, CD-roms, a glossary etc.
• EUROPEAN VOLCANOLOGICAL SOCIETY with eruption updates, description of the Decade Volcano program, general and specific articles regarding evolution in the future - hazards assessment of selected individual volcanoes.
• Société de Volcanologie de Genève (Suisse) (in French) - The Swiss Volcanologic Society's web site with general information about about itself, its members, travel proposals, links. The SVG publishes a nice printed monthly bulletin with news and information about volcanoes.

• Santorini Decade Volcano (part of this site)
• METHANA A large and comprehensive site dedicated to Methana (a volcanic peninsula close to Athens) and other regions of Greece. Made by photographer Tobias Schorr, it explores geology, nature, archeology, life, turistic attractions and more. (So far only in German and Greek). 

• ITALY'S VOLCANOES "THE CRADLE OF VOLCANOLOGY"!!!!. Boris Behncke's phantastic site on Italian Volcanoes. Enthusiastic, comprehensive presentation and description of all major Italian volcanoes and volcanic areas with maps, photos and reference lists, and links for further information. Focuses on ETNA, STROMBOLI and VESUVIUS with very detailed information about past and current activity. EXCELLENT LINKS to other volcanic sites.
• Charles Rivière maintains a website (in French), extremely up-to-date, with detailed information about Etna's activity and latest results on geochemical studies on recent Etna lavas conducted by research team around R. Clocchiatti..
• Istituto Nazionale di Geofisica e Vulcanologia (I.N.G.V.) The official site (with its hopefully finally stable URL) of the volcanological research institute in Catania with news and scientific reports on Etna, Stromboli and other Sicilian volcanoes. 

• THE NORDIC VOLCANOLOGICAL INSTITUTE (NVI) Iceland's volcanoes. News and photographs from the ERUPTION AT GRÍMSVÖTN VOLCANO, VATNAJÖKULL ICE CAP, 18 - 28 DECEMBER.
• Geology of Iceland - a concise, concentrated, and beautiful introduction to Iceland's geology with a large number of references, photos and graphics by Steve Lonker
• Iceland volcanoes - summarizing information about Iceland and its volcanoes, lots of links and more detailed information on selected topics, nicely organized by USGS

• ALASKA VOLCANO OBSERVATORY Great site with frequent updates. 

• CHILE'S VOLCANOES

• VOLCANO WATCH - The Hawaiian Volcano Observatory. Info and news about Hawaiian volcanoes.
• HAWAII CENTER FOR VOLCANOLOGY - Comprehensive information on Hawaiian volcanoes, links to research centers

• SOUFRIERE HILLS, MONTSERRAT - The Montserrat Volcano Observatory

• PINATOBU

• VOLCANOLOGICAL SURVEY OF INDONESIA. Updates and information on Indonesian volcanoes.

• COLIMA (In Spanish.)
• POPOCATEPETL (In Spanish.) Contains detailed information, daily updates and a live-cam.

Ol Doinyo Lengai, Tanzania: Links
 

• EUROPEAN VOLCANOLOGICAL SOCIETY with very detailed and usually daily eruption updates. Has grown into probably the most comprehensive one on the net.
• VOLCANO DISCOVERY's news section with frequent updates on volcanic activity worldwide
• PRELIMINARY NOTES ON VOLCANIC ACTIVITY The Smithsonian Institution Global Volcanism Program. The best source for ongoing activity, now divided in sections with eruption updates from all the world.
• YAHOO!NEWS
• Check also national and regional newspapers ! A good list is provided by Yahoo! here.