Below is a list of some of the better butterfly (and some other) websites as compiled by myself. If you have a website that you would like to be included here, let me know!
European Butterflies and Moths: an extensive website, with new images and species added constantly by many contributors. Huge number of superb live pics, including pre-imaginal stages. A must-see.
Matt's European Butterflies: run by Matt Rowlings, a strong contender for my all-European favourite. Digital photos and older video stills. Many eastern species - a major plus in my view!
Captain's European Butterfly Guide: among the best-known general websites for European butterflies, authored by Simon Coombes. Live photos of 182 of Europe's 440-plus species, with a brief account and map for most of these.
Mario Maier's European Butterflies: a partly bilingual (German and English), extensive and very well made website with photos of many species.
Guy Padfield's European Butterfly Page: another interesting website with lots of live photos.
New: European Butterflies by Chris Jonko: extensive site in three languages (English, Polish and German) which also includes moths, with numerous photos by many contributors.
New: Butterflies of the Western Palearctic by Jaroslaw Bury: in Polish. Scroll down to the galleries of separate families (on the left of the screen).
New: European Butterflies and Skippers by Juha Tyllinen: in English, features distribution maps and photos of set and/or live specimens by several contributors.
Europe by country
Austria: in German only, with lots of very good photos by Johannes Gillmann et al.
Benelux (Belgium, the Netherlands and Luxemburg): all species occurring in this region pictured on colour plates scanned from 'Dagvlinders van de Benelux' (Wynhoff, van der Made & van Swaay 1990); info on distribution, abundance, larval host plants etc.
Bulgaria: Dimitre Staykov's The Insects of Bulgaria has a checklist of the butterflies of Bulgaria with information and photos/illustrations of some species.
Cyprus: Eddie John's excellent website presents all butterflies of this fascinating island with excellent photos and locality data.
Finland: there are two extensive websites dedicated to the butterflies and moths of this country: one by Arto Avanto, the other by Pertti Pakkanen. Both are frequently updated observation logs, with lots of images and distribution information.
Greece: Checklist of all 235 species of Greek butterflies, with brief information on the distribution and photos of set specimens with locality data for most species.
Great Britain: here is one website with set specimens of all species - resident, frequent immigrants, rare strays and imported exotics - but little textual information. New: the UK Butterflies website is one of the most comprehensive source on the butterflies of any country and has a plethora of features and information - including a brand-new report from the 2007 butterfly-watching trip of the British-Bulgarian Friendship Society to Bulgaria.
Hungary: text for most species but unfortunately in Hungarian only, and photos of many species in nature and/or of set specimens.
Ireland: contains common names only.
Italy: a cryptic but otherwise good website in Italian only with high-quality photos of set specimens of all of Italy's butterflies, and images of pre-imaginal stages and host-plants for some. This country's butterfly fauna is the richest in Europe (270+ species), so this site is worth checking out even if you hate to see pinned specimens. On the downside, the taxonomy used is extremely split-up and what is in my opinion one species has in several cases (e.g. some Lycaena, Pyrgus) been divided into several 'species'.
Norway: a well-designed website with both live and pinned images of each butterfly species.
Serbia: The website of Milan Djuric has a checklist of the butterfly species of Serbia with good photos of live specimens. New: The website of Ivan Dodok lists and illustrates Serbian butterflies and moths, with an emphasis on the western part of the country.
Ukraine: The most interesting region in the country, butterfly-wise, is the Crimean Peninsula, and now its butterflies (and moths) have their own website by Vladimir Savtchuk. Detailed information on the Peninsula and its habitats and numerous live photographs of almost all butterfly species, including some remarkable local endemics. Sergey Popov's website deals with the monitoring of the butterflies of Western Ukraine (the Carpathian region). Detailed distribution maps and also photos for some species, but unfortunately lacking in quality.
Turkey: the website of Ahmet Baytas so far presents a checklist of Turkish butterflies with brief notes on range, habitat and flight-time, and photos of some species.
Butterflies of the lake Baikal region. Oleg Berlov's Russian-only website has excellent images of set specimens of all species to be found in the region of the world's deepest lake.
Siberia and Central Asia: a website by Oleg Kosterin featuring butterflies, spectacular scenery, plants and some other insects. Each image is accompanied by very detailed locality data.
Japan. As to be expected of the country where almost all of the world's camera manufacturers claim residence, there are numerous websites with butterfly images. Here are a few examples: Takashi Kumon's site (very large, incl. a subsection on the butterflies of Hawaii); Masamitsu Mori's site (large, well-organized); Mochimitsu Kobayashi's site (smaller than the preceding, apparently covers only the small area of Kiso in central Japan).
Morocco: there are two stylish websites in French only, dedicated to the nature of faunistically perhaps the most interesting region of northern Africa, very rich in endemics. The first has very little information but lots of beautiful butterfly and other photos by Michel Tarrier and Jean Delacre. The second is M. Tarrier's own site.
Butterflies of Canada: a superb website with detailed information, incl. map, for each species and color photos of specimens (pinned, some in nature) for most of them. Based on the book of same name by Layberry, Hall & Lafontaine (1998).
Butterflies and Skippers of North America: an excellent, extensive and well-organized website with lots of photos for identification purposes.
Butterflies of North America: a very similar website to the preceding one (same authors), but designed and organized in a somewhat different way, with live photos only.
Southern Australian Butterflies: very comprehensive, excellent photos, illustrated identification keys.