Here are a list of Running Gags in Aruseus Emails.

Season 3-4

Husk Cameos

Husk may be mentioned or seen in the background of Arumail.

summer 2008: A Husk doll is visible in the crane game next to the one Dan is playing.
dumb plans: Husk is seen at the bar behind Aruseus and Ryan.
football: In the scene where Ryan is playing Power Battle, there is a Husk arcade game next to him.
Airman?: In the scene where Wolf is being attacked by the Fan Fiends (fat guys with fans on chest), Husk is seen far away being attacked by Pipis (The boids).
birthday: Husk is one of the guests.

This all started when I ment to type crap, but typed carp instead. Ever since, it has been commonly used.

catchphrase: Aruseus says, "Carp. I mean crap!" and then later says, "Double carp."
cereal: Aruseus says, "Oh, carp. Gotta run."
shoe: Aruseus exclaims, "Holy carp!" after Foxx's head explodes.
nemesis: Aruseus exclaims, "Holy carp!" once more.
triplemail part deuce: Aruseus asks which email was carpmail, when he meant to say crapmail.
Virus Reloaded: Lyle says, "Holy carp..." before Aruseus turns human, and Dylan states to Foxx "why the carp" he's human.
summer: Aruseus says, "Holy carp-" before Strong Mad cannonballs in the pool.

Dance Dance Revoltuion is mentioned in Arumail.

summer plans: The email song is a parody of BREAK DOWN! from DDRMAX 2. Also, Aruseus is seen playing DDR at the end. The song A Stupid Barber from DDR Supernova is also played when Aruseus mentions his 5 favorite things to do in the summer.
systems of gaming: The email song is a parody of Crash! from DDR Supernova. Aruseus says DDR is one of his favorite games.

Decapitation is sometimes mentioned in Aruseus Emails.

evil: Strong Bad threatens to kick Kyubii's head off.
oddities: Fang pulls his head off to show the viewers what he can do.
different bodies: Kyubii asks Fang to decapitate him to fufill his dream of being a head in a jar.
summer: Strong Bad asks Aruseus if he got decapitated.