(This is not an exhaustive list...)
Craig Davey- UK ~ excellent painting guide
Peter Dunn - NZ ~ eye-candy site with lots of delicious high quality photos of figures
Firefall - NZ ~ the best collection of battle-reports on the web (IMHO).
Nik Gaukroger- UK ~ useful DBM-dedicated site.
Matt Haywood UK ~ a site dedicated to warfare in Eastern Europe.
David Kuijt- USA ~ a very useful figure-review page.
Peter Page- NZ ~ nice Kiwi site
Alan Patrick - UK ~the invaluable slacker's guide to painting, how to design armies and how to use lots and lots of knights.
Mark Plant- ex NZ ~ Pygmy Wars- Wargaming the Russian Civil War
Tim Porter UK ~ humorous site with some good accompanying pictures.
Miniaturewargaming ~ excellent general wargaming blog by John Retzer.
Francis Small USA ~ a site dedicated to the Medieval French.
Luke Ueda-Sarson - NZ/Japan ~ a site specialising in Hellenistic warfare but with plenty of other goodies.
David Young- AUS ~ Australian DBM Rankings
Jeff Zorn - USA ~ includes some stuff on Crusades and how to take photos of miniatures.
Urban Warfare- NZ based retailer of figures, rules and accessories. Keen supporter of local wargaming.
The companies below produce figures and equipment for wargaming in this period. In most cases I recommend getting samples before committing to large purchases- and doing some of your own research first. This is especially true for the case of Ghulam or Mamluk cavalry. It is not always prudent to presume that manufacturers have thoroughly researched all their ranges. David Kuijt maintains a very useful figure review page.
Outpost produce the most accurate ghulams (I commissioned them). They plan on expanding their range to cover the whole Crusading Era. This means that finally, accurate Byzantines may come available.
Essex- true 15mm figures and probably the widest range around. The figures are well cast with good detail but sometimes lack a dynamic stance. Some Asiatic cavalry have bowcases placed backwards and horses may be generic rather than Turco-Iranian. Essex also stocks a wide range of buildings, terrain, rules and paint.
Gladiator- a good but older Outremer range of Franks, Saracens and Byzantines. One of the few manufacturers to produce Saracen cavalry with plenty of maces and swords. The figures tend to have outsize hands and cuffs. They are also larger than true 15mm. Their Feudal range is quite accurate for Western troops and closer to true 15mm.
Museum- Museum has two basic sizes for their figures. Their Norman and Islamic Persian ranges are relatively large and are hard to mix with other manufacturers. Their Arab and Mongol ranges are closer to 15mm. Their Normans and Crusaders are generally accurate. Their Asiatic cavalry can have serious lapses in historical accuracy.
Donnington- A large range of Outremer figures. Figures vary in their casting and sculpting quality. Some are excellent and some are very basic. The figures tend to be sculpted in a simple style with little extraneous equipment such as belts, bags etc. Hands tend to be typically smaller than the other ranges. They are close to true 15mm, hence smaller than the large Museum or Gladiator figures. Generally compatible with Essex figures, but more accurate for ghulams.
Irregular- Produces a large range of 15mm figures. I have never purchased from this range so can not comment on their quality.
In Australasia, Eureka Miniatures can supply the Hallmark, Museum Miniatures and Irregular Miniatures ranges.
Veni Vidi Vici shield decals, handy for those of us who find painting crosses to be an arduous exercise in practical geometry.
A more complete range of manufacturers can be found at The Miniatures Page