Me, when I play Veigar every time I get a kill with his ulti or Dark Matter I type,
"WUT DUH FKKK BOOOOOOOOOOOOOOM!" in all chat. In fact, I have it copied and pasted when I play him.

I also tend to have a habit of calling myself middlesticks, feeeedlesticks (not because I feed but just say it out loud), or whatever sort of name when I play fiddle.

Now it's your turn.

i spam in all chat how beautiful my butterfly kog is

I have an idiotic habit of repeating everything Gangplank says.

Right Click
Me: YAR HAR HAR
Right Click
Me: AVAST

I don't type it or anything. I just annoy my skype group by speaking like a pirate.

Fortunately I don't play GP much.

I wouldn't say it's a habit, but during my last game as Lux I screamed "DEMACIAAAAA" into the mic while firing mah lazor.

Everyone on Skype burst out laughing.

Needs moar crittlesticks

Also, ROCK SOLID into all chat when ever I dive someone as malph.

When I kill someone with my Karthus ult

I have to perform and IRL Dance of Death
which takes 9-10minutes to complete.

During which I am AFK, wherever I happened to get the kill.

Then I smear myself in goat blood and continue the game.

I go AD+Smite with Soraka.

Person: KS OMG!
Me: I'm Lee Sin, every kill is mine to take.

I can't stop with the Lee Sin jokes...

I use "It's only fun if they run" at least 10 times/ ww game

i walk up to enemy towers as the only person in range of them and /t them as mundo, calling them sissies while they fire at me