VintageCalc - A delay-line memory calculator on an FPGA

Some of my favourites:

Old Calculator Museum - by Rick Bensene

A comprehensive website devoted to preserving, documenting, and sharing the technology of desktop automatic calculating machines -- from the electro-mechanical calculators of the 1950's and 1960's, through the beginnings of the pocket wonders we have today, which had their roots in the early 1970's, and on into the beginnings of the desktop computer of the mid-1970's through the early 1980's.

Vintage Calculators Web Museum - by Nigel Tout

Award-winning site of an 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.

Calculator Simulations - by Brent Hilpert

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!

BE-Calc - by Serge Devidts

A great site put together by a collector in Belgium. Detailed photos and technical information on old electronic calculators.

DIY Calculator - by Clive Maxfield

An interesting teaching approach using a custom-designed virtual machine.

Many more calculator-related sites are listed here.