Here is a list of ones that I like...
AxisAllies: Spring 1942
Husted’s Variant Rules Compendium
Mechanized Infantry: Infantry Paired With Tanks
Each infantry unit paired with a tank may blitz with the tank. Infantry not paired with a tank may not blitz. Both the tank and infantry paired together must start in the same territory and move to the same territory.
Airborne Infantry: Infantry Paired With Bombers
Each infantry paired with a bomber may "blitz" two territories on the attack. The bomber and infantry must both start in the same territory and the bomber must end its combat move in the same territory as the airborne infantry. This ability to combat move two territories includes the ability to pass over a sea zone (without regard to units in the sea zone).
Marine Infantry: Infantry Attacking After Naval Transport
Each infantry unit attacking after being transported by a naval transport, during the same turn as the attack, hits on a 1 or 2 for the first round of combat (one roll of the dice).
Combined Arms Bonus: Fighters Paired With Tanks
When fighters and tanks are attacking in the same territory, for each fighter that is paired with a tank, the fighter attacks at 4 rather than 3.
Armed Transports: Liberty, Victory, & Q-Ships
Transports alone in a sea zone, with no friendly or allied warships present, defend at a 1 for the first round of combat only (one roll of the dice), but may not attack.
Attacker and defender declare which, if any, of their artillery is Directed Artillery before combat. Directed Artillery attacks are resolved first.
• Directed Artillery is not paired with infantry and gives no attack bonus.
• Directed artillery is rolled separately and hits on a 1 or 2.
• On a roll of 1, the rolling player selects the casualty.
• On a roll of 2, the owning player selects the casualty as usual.
Let me know what you think! Let others know what your favorite rule variants are. Have fun!