Mark Glusker's "Mechanical Calculation" site
Mark has put together a very nice site that focuses on later model electro-mechanical calculators. He has an impressive collection of wonderful machines from Friden, Monroe, Marchant, Olivetti, and others. He also as a great section on his amazing development of a re-creation of Thomas Fowler's ternary (Base 3) calculating machine of 1840.

"Funky Goods" Calculator Museum
A nice collection of various electronic calculators from the time period of 1966 to 1974. Site is in Japanese. Homepage translates well into other languages with Google Translate, but unfortunately, at least as far as I can find, the individual calculator pages do not seem to translate properly. Detailed nice-quality photos of calculators inside and out.

Virtual Museum of Calculators (Virtuális számológép-múzeum)
A stunningly well crafted, beautifully photographed, and very informative site about calculating instruments, located in Hungary. Site is in Hungarian, but also presented in English. Link goes to English version of the site. A large collection (over 300) of electronic calculators, including some fascinating early Hungarian-made calculators, including Hungary's first electronic calculator, the Hunor 131, and the Intel 4040 microprocessor-based Hunor 301 Programmable Scientific desktop calculator. Also has great exhibits on slide rules made in Hungary.

Robert Lacoste's "Calculatrices" site
French collector Robert Lacoste has an impressive collection of calculators dating from the 1920's through 2010. Great photography of the calculators. Site is in French.

John Wolff's Calculating Machines Web Museum
A very fine site, put together by Australian collector John Wolff, that documents many aspects of calculating machine technology, from slide rules to electronic calculators. High quality photography and extremely well-written exhibits. Of special note is an extremely detailed account of the workings of an electro-mechanical work of art, the Madas 20ATG.

A Collection of Mechanical Calculators
A nicely presented site that presents a collection of mechanical and electromechanical calculators from "Before the Electronic Age".

Katie Wasserman's Calculator Manual Repository
A great and growing collection of PDF scans of entire calculator user's manuals.

Daniel Sancho's Virtual Museum on the Evolution of Calculating Machines
A very nicely prepared site, with wonderful documentation on the history of calculating machines, including early mechanical and electro-mechanical adders and calculators, early electronic calculators, and later evolution of calculators and computers. Site is mostly in Spanish, but, for those who are not literate in Spanish (such as myself), Google's web page translator clearly shows the quality of Daniel's research.

Brent Hilpert's EEC - Early Electronic Calculator Technology Reference
Truly amazing site with reverse engineering and simulations of old electronic calculators. Lots of detailed data on obsolete integrated circuits that are used as the guts for calculators from the late 1960's and early 1970's. A must-see!

Laura & Michael Greenfield's Vintage Technology Museum
Vintage technology through the years. A wonderfully-designed site, with a time-machine theme. Includes calculating machines, along with radios, television, lab equipment...you name it, it's probably there.

Joerg Woerner's Datamath Calculator Museum
A site specializing in exhibition and documentation of classic Texas Instruments calculators, along with those that use Texas Instruments IC's as their brains.

Nigel Tout's Vintage Calculator Web Museum
Award-winning site of a fellow old calculator collector in the UK who does a great job of preserving and documenting old calculating machines. Special interest in machines made in the UK.

Dentaku Museum
A wonderful site documenting the history of Japanese electronic calculator technology. Lots of interesting information. Some pages in Japanese only, but most pages also have english text. Some great photos of early Casio relay calculators.

Serge Devidts' Calcuseum
A fantastic site put together by a fellow collector in Belgium. Detailed photos and technical information on a great many old electronic calculators.

Museo Nazionale degli Strumenti per il Calcolo
An Italian National Museum devoted to documentation and preservation of calculating instruments, located near Pisa, Italy.

Francesco Bonomi's Olivetti Programma 101 Information Source
An extremely well presented collection of information on the Olivetti Programma 101, one of the earliest all-electronic programmable calculators.

Mike Sebastian's Calculator Technical Information Web Pages
A site specializing in information on early LSI chipsets used in calculators.

Sergei Frolov's Soviet Digital Electronics Museum
The Soviet Union had some truly amazing calculator technology. Sergei's site documents an ever-growing collection of Soviet calculating technology. A definite must-see.

Gerhard Wenzel's Museum of Pocket Calculating Devices
A comprehensive and amazing collection of pocket calculators, along with many other calculating devices. Great photographs.

Rick Furr's Calculator Reference
A site packed with information on Texas Instruments, Hewlett Packard, and Curta calculators. The definitive source for information on Curta calculators.

David Hicks' Museum of HP Calculators
The definitive web resource for information on Hewlett Packard calculators. Very high quality web design, with tons of useful information. Can't recommend this site highly enough!

Kevan Heydon's Calculator Pages

Bruce Rogers' Calculator Collecting Pages
Bruce documents his nice collection of mainly Hewlett Packard and Texas Instruments calculators, along with a number of other machines. Bruce also collects cameras, test equipment, video game systems, and model rockets.

Alex Knight's Electronic Calculator History and Technology E-Museum
A fellow collector from the East Coast of the US who shares the same vision for preservation and documentation of old electronic calculators as I. Lots of great articles and photos of early calculator technology. Gotta love those early Mathatron calculators in his museum. Check it out!

Dan Veeneman's "Decode Systems" Calculator Collection Site
An informative and nicely produced site featuring Mr. Veeneman's great calculator collection.

Early Office Museum
A museum devoted to documenting and preserving all equipment used in offices. Great exhibits on things like fastening technology, copying technology, and even great detail on pencil sharpeners!

Yesterday's Office
A beautifully crafted site specializing in anything realting of vintage office equipment, including typewriters, duplicating machines, cash registers, adding machines and calculators. Features articles on various museum sites around the world.

Mike's Electric Stuff
Check out the page on Nixie Tubes, Counter Tubes, and Selector tubes. Great stuff!

Wang2200.org
Jim Battle has developed a fantastic site dedicated to Wang's early 2200-series of BASIC computers, that were an outgrowth of Wang's high-end calculator designs. Beautifully-designed site, extremely well-written, and loaded with great historical and technical details. Also features a Wang 2200 emulator that runs under Windows or Macintosh that executes the actual Wang 2200 microcode captured from an early Wang 2200 computer. This is about as accurate as an emulation can get!

The Small Wang Museum
A wonderful museum devoted to the machines of Wang Laboratories, put together by Jan Van de Veen of the Netherlands. Jan has a very highly-optioned Wang 720C calculator system, as well as a great selection of early Wang computer equipment.

The Computer Museum History Center