The Library of Congress has a gallery of World War I posters. I like to spice up my gaming terrain by printing out period posters and sticking them on the walls of buildings.
The Great War In A Different Light site has an amazing collection of color photos from World War I. These are not black and whites that were colorized after the fact, but original color pictures.
Mike Cooper has photos of several World War I dioramas that he has made. There’s Austrian Artillery, one of the Great War in Palestine, a trench mortar and a gas attack model.
Whether you’re into Steampunk, Pulp Adventures or WWI, sooner or later you’re going to need an armored car. Here’s one you can build out of paper in 25mm - 28mm scale.
To go along with the previous post on Red Actions!, the free Russian Civil War rules, here is the Jackson Gamers' RCW site. There's plenty here to look at!
Here's are some instructions on building a trench system for miniature wargames set in World War I. I also think that they would be useful for the Warzone science fiction game, which has a definite WWI flavor. This period has experienced somewhat of a renaissance lately, with several companies, including Renegade and Brigade games releasing 25mm - 28mm wargames figures.
Rudi Geudens has a page with photos of World War One guns and tanks from the Belgian Royal Museum of the Army and Military History in Brussels. Photos such as these are always useful when painting your wargames miniatures.
Miniature Wargaming is part of the "adventure games" hobby, which includes r ole p laying and board games. Wargamers recreate battles on the tabletop with toy soldiers, like a more complicated game of chess. Models range in height from 6mm to 28mm tall, with 15mm and 25mm being the most popular. There also is a growing interest in toy soldiers and military models, such as the 1/32 and 1/35 scale plastic soldiers from Conte, and Marx.
The most popular miniature wargames are fantasy and science fiction based, such as Warhammer, Warhammer 40K, Warmachine and The Lord of the Rings. World War II games such as Flames of War and Axis and Allies are new favorites. Other favorite historical periods include Napoleonics, the American Civil War, and ancients, such as Romans or Greeks. Other gamers enjoy miniature naval wargames, recreating battles like Trafalgar, Jutland and the Coral Sea.
Hobbyists research historical periods and paint their tiny soldiers in accurate uniforms. Others develop "historically realistic" rules sets or build scale battlefield terrain using model railroad techniques.
For pictures, visit the gallery.
Some of the bigger hobby companies are Games Workshop, which produces Warhammer, Wargames Foundry and Old Glory Miniatures. Wizards of the Coast produces several lines of pre-painted miniatures games, such as the Star Wars and Dungeons and Dragons miniatures games, and a historical game with pre-painted miniatures: The new Axis and Allies game. Wizkids produces a fantasy collectable miniatures game, such as the Mage Knight and Heroclick fantasy games, the science fiction games MechWarrior and Rocketmen, as well as the quasi-historical Pirates of the Spanish Main.