Another classic variant from kent.ac.uk Risk FAQ. Though I’ll classify it under House Rules since it is quite simple and it can be easily combined with many different variants and house rules.
Under this variant, any territory can attack any other territory even if they are not adjacent. The attacking territory immediately loses half its armies (rounding the loss up), then the attack proceeds as usual. If the attacker breaks off the attack to conduct a different attack, including attacking the same target from another location, the airlift is over and resuming the airlift-based attack requires again losing half the remaining armies.
Variants on the variant include limiting each player to a single airlift attack per go, and/or requiring that the player be holding the card for the target territory.