Antique scientific instruments may not be the first things that come to mind when the word technology is mentioned, but many were, in fact, key technologies of their day.  Many of the instruments that played major roles in the development of science and in its application to practical problems were related to the study of the heavens and to timekeeping.  The astrolabe, for example, which is more than 2,000 years old, provides a means of showing how the sky looks at a particular place and time. This allows a user to determine the time of day (using the sun in daylight or a particular star at night), the time of sunset or sunrise (and so, the amount of daylight remaining). It can also be used to help identify specific stars which could then be used for navigation.

Telescopes and microscopes have been in use since their invention around the turn of the 17th century.  Their development from the simple instruments used by Galileo to the often large and complex opto-electronic devices of today is a chronicle of the progress and interrelations of science and technology.  Many of the museums that house collections of historic scientific instruments have put these collections on line in recent years.  Optical instruments, as well as antique astronomical devices (armilliary spheres, orreries, etc.), and medical devices are popular with collectors and an active market exists, as can be seen from some of the sites linked below.  Less popular, but no less interesting or important in the history of science and technology are antique instruments from the fields of chemistry, mineralogy, and electrostatics, a few of which can be found through links at the bottom of the list.

Links:
= highly recommended

Epact--A web-based catalogue of 520 medieval and Renaissance scientific instruments set up by the Museum of the History of Science at Oxford University in collaboration with three other museums in the United Kingdom, the Netherlands, and Italy. The photos are outstanding and there is a great deal of descriptive, explanatory, and bibliographic information.

Reflections from Alchemy to Astrophysics--An exhibition of historic scientific instruments from the School of Physics and Astronomy at the University of St Andrews (Scotland). Includes an explanation of the origins and function of the armilliary sphere pictured above, which was made by Humphrey Cole, a London instrument maker who lived in the 16th century.

American Artifacts: Scientific, Medical and Mechanical Antiques--American Artifacts is a journal "devoted to entire spectrum of scientific, medical and mechanical antiques." The site contains some articles from the journal, as well as instruments for sale, links and other miscellaneous stuff. Interesting, but not easy to navigate.

Antique Telescope Society--an organization that brings together people interested in antique telescopes and related items, and promotes interest in astronomical history and discovery, the history of optics, and the preservation and use of the antique instruments. Its 2001 convention is in Pittsburgh, PA, in September. The site also includes a Virtual Museum of the History of the Telescope.

The Astrolabe. A site devoted to this "very ancient astronomical computer for solving problems relating to time and the position of the Sun and stars in the sky." Among the interesting things that can be found here is an "Electric Astrolabe," a computer based version of this instrument, that is available as a free download.

"Churches as Scientific Instruments," an article on J. L. Heilbron. "During the 17th and 18th centuries cathedrals in Bologna, Rome, Florence, and Paris served as centers of astronomical investigation - or, to speak in our overheated language - as state-of-the-art, world-class, solar observatories."

Science Treasures--An Oklahoma City-based dealer in antique scientific instruments. 

Naturalia et Artificialia--An Italian handcraft firm that makes beautiful reproductions of antique scientific instruments. (Site is in English as well as Italian.)

ars machina.com--Antique microscopes, telescopes, globes, and other instruments and artifacts. This is a dealer site, but has "museum" sections loaded with information on the history and development of microscopes, telescopes, etc. The photographs of these instruments are outstanding.

Some sites featuring other types of antique scientific instruments:

Scales and related devices
Glassware and other chemistry equipment 
Mineralogy and gemology equipment
Meteorology
Electrostatic machines (if you've never seen a double Wommelsdorf machine, here's your chance)
On-line catalogue of alchemy supplies (not really antique, just strange. . .)