Illusions and impossible figures

http://www.illusionworks.com
An old web project about illusions by Al Seckel (English)

http://planetperplex.com
Very large collection of images with illusions and impossible figures. Many pictures by famous artists. (English and dutch languages)

http://www.sandlotscience.com
A large site about illusions and impossible figures with animations and Java-applets. (English)

http://www.kfki.hu/~nyikos/3d/00.html
Large collection of impossible figures (Hungarian language)

http://figuresambigues.free.fr
Large website about illusions (French)
There's kindred site about fictitious figures at http://figuresfictives.free.fr

http://www.log-in.ru
Large collection of illusions and impossible figures including (Russian)

http://www.lookmind.com
A site about illusions - an english variation of the site above (English)

http://www.anopticalillusion.com NEW
Blog about optical illusions by Brad Honeycutt

M. C. Escher

http://worldofescher.com
Big gallery with Escher's artworks, competitions, electronic shop etc. (English)

http://www.mcescher.com
Official M. C. Escher website (English)

http://www.mcescher.net
The one of the biggest collection of Eschers's artworks in Internet (English)

http://users.erols.com/ziring/escher_bio.htm
Detailed Escher's bigraphy (English)

http://www.mathe.tu-freiberg.de/~hebisch/cafe/mce/escher.html
Collection of rare Escher's artworks 

Oscar Reutersvard

http://www.caldarelli.it/oscarreutersvard.htm
Much original images by Oscar Reutersvard. (Italian language)

http://members.tripod.co.uk/brisray/index.html
Big collection of Escher's, Sandro del Prete's and Reutersvard's images. (English)

http://www.limegrafik.se/grafiska/reutersvard.htm
Few images by Reutersvard. (Swedish language)

http://www.palmyria.co.uk
A site with various Reutervärd based images. (English)

Sandro del Prete

http://www.del-prete.ch
Official Sandro del Prete's site. (English)

http://mitpress.mit.edu/e-journals/Leonardo/gallery/gallery331/homageescher.html
Leonardo On-line gallery. Collection of images by Sandro del Prete, Jos de Mey, Istvan Orovits and others. (English)

Istvan Orosz

http://www.geocities.com/SoHo/Museum/8716
Istvan Orosz's website. (English)

http://www.jatekhaz.skicc.hu/htmlm/gal/3908orom.htm
Three artworks by Orosz with well quality. (Hungarian language)

http://www.marlenaagency.com/Pages/orosz/orosmal.html
Extensive collection of images by Orosz. (English)

Miscellaneous

http://www.irvinepeacock.com (NEW)
Official website of Irvine Peacock (English)

http://www.rakov.de/(NEW)
Website of russian artist Dmitry Rakov. (Russian and english languages)

http://opart.narod.ru/ (NEW)
Website of russian artist Igor Achkasov (Russian and english languages)

http://pubweb.northwestern.edu/~gbuehler/index0.htm
Website of german artist Guenter Albrecht-Buehler (English)

http://www.arsetmathesis.nl/
About mathematical art. (Dutch language)

http://wwwtbw.mfh-iserlohn.de/informatik/devries/spielwiese/unmoeglicheObjekte/english/Reality.html
A small book about impossible figures. (English and german language)

http://www.kv.by/index2000464401.htm
Article at electronic magazine "Computer News On-line" about illusions. (Russian language)

http://www.mrmagician.co.uk
Website focused on how to learn magic.