Below find categorized links to web sites about cacti and succulents

Informational and picture sites

Desert Tropicals - A site run by a French fellow in Phoenix. Here is a huge database of information on gardening in the southwest.
Daves Garden-Gardening of every type, a huge plant database, forums and more.
Cacti Guide-Information, resources and pictures of specimens in nearly every Cacti genera. Some nice clear pics of gorgeous specimens! Also a great forum.
Xericworld - An on-line community for Xeriscape gardeners. Lots of good information and comradery here.
Cactus and Succulent Plant Mall - Sounds like an on-line nursery but actually it's a pretty good resource site with more info and links than you can imagine.
Botany.com - An on-line Encyclopedia of Botany.
The Succulent Plant Page - Very good info on cultivation. Very well done!
Au Cactus Francophone - A french site translated to English. Excellent info and nice pics.
Kaktuskasvit - A foreign page thats basically an index of Google searches for cacti pictures.
Cactacea.net - A giant database of cacti and succulent pictures from European enthusiasts. Over 10,000 pics!
Cactarama - Another picture site. Some nice pics!
PK's Cacti and Succulent Pages - An amateur photographer and gardener from Australia shares some of his hobbies and pictures.
Desert Ecology - A very informative site, not too many pictures though.
Cacti@Purlins - Pictures of the collection of Philip Andrews from the U.K.
Cactus Art - Though basically an on-line nursery, I've included it here because the information and photography is outstanding!
The Amateurs Digest - Excellent info site on Cacti and Succulents Some nice pics!
Cultivar - An e-magazine from Russia that highlights crested, monstrose and variegated cacti. Very well done!
Diversity of Life.org - A searchable database of flora and fauna of our planet. Huge!
Haworthia.com - Everything you could know about Haworthias.
Pachypodium.net - Everything you could know about Pachypodiums, the Madagaskar Palm.
The Exhibition - An outstanding index of photographs from Chris Moore's Asclepiadaceae collection.
The Lohmüller Homepage - Homepage of German photography team Andrea and Friedrich A. Lohmüller. With a beautifull cacti and succulent section.
Mammillarias.net - Well done site devoted to the genus Mammillaria.
Stapeliads "Orchids of the Succulent World" - Some breathtaking photos of Stapeliad flowers.
Euphorbia.de - Great site about Euphorbias from Frank Vincentz of Germany.
Astrophytum - Great site about Astrophytums from Heinz Hoock of Germany.
Ariocarpus "Living Rocks of Mexico" - The most comprehensive resource on the genus Ariocarpus available.
Agavacea - A site documenting the genus Agave in habitat.
Lithops World - Nice site about Lithops.
The Adenium Page - All about Adeniums, the "Desert Rose".
Lithops: An Introduction - Another good Lithops site.
Gasteria Reference Collection - All about Gasterias.
The Genus Thelocactus - Good page about Thelocactus.

Cacti and Succulent Societies

Tucson Cactus and Succulent Society
The Cactus and Succulent Society of America
The British Cactus and Succulent Society The Cactus and Succulent Society Of Australia
On Line Nurseries

Arid Lands
Shoal Creek Succulents
Squaw Mountain Gardens
Out of Africa
Natures Curiosity Shop
Living Stones Nursery
High Country Gardens
Botanical Gardens with Cacti and Succulent Sections

Tucson Botanical Gardens - Tucson, Arizona
Arizona Sonora Desert Museum - Tucson, Arizona
Boyce Thompson Arboretum - Superior, Arizona
Desert Botanical Gardens - Phoenix, Arizona