There aren’t many resources online for geomancy online, and a Google search for “geomancy” will turn up twenty sites on feng-shui and ley lines for every site on geomancy itself. The following links are
Part of an extensive site on divination methods, the Adula geomancy page presents interesting interpretations of the geomantic figures.
A useful article on the basics of geomancy by horary astrologer Anthony Louis, this is a good introduction to the art.
A sprawling online collection of texts on the I Ching and geomancy, mostly in French or Latin, but including English versions of Agrippa's On Geomancy and Gerard of Cremona's On Astrological Geomancy. Not the easiest site to use, unfortunately.
A moderated public email group (apparently the only one on the net) for students and practitioners of geomantic divination, Geomantic Campus features lively but polite discussions on geomancy and analysis of geomantic charts.
This site features an online geomancy program that casts geomantic charts, and provides basic information on the geomantic figures and their interpretation.
Aleister Crowley's reworking of the Golden Dawn geomancy system, as published in his periodical "The Equinox". Useful mostly as a look at the sort of “look it up in the tables” approach to geomancy that was responsible for making the art the forgotten oracle of the western world.
A fine site on medieval geomancy created by Elizabeth Bennett, a scholar of medieval history at Princeton. Includes instructions for casting a geomantic chart, a bibliography of sources, a simple geomancy program, and Cornelius Agrippa's classic On Geomancy.
For those who are looking for the "other geomancy" - ley lines, feng shui, and dowsing - a great site with information on every aspect of earth mysteries studies and plenty of links to other resources.
This excellent site on astrology and magic created by traditional astrologer Christopher Warnock offers several of the most important Renaissance geomantic texts on CD, and his geomancy computer program, Geomanticon, is in beta testing right now and will be available for sale shortly.
Dutch anthropologist Wim van Binsbergen has done extensive research into geomancy and related African systems of divination, and some of his papers on the subject are available at this site. Well worth reading.