Some common ones are Less than three <3, a sideways heart <3 (normally meaning love), and a face <3.
The < is the eyes/eyebrows, in a.. well, happy position. Kinda. And the mouth, 3, is like an animal mouth. Kinda.
Such as :3, =3, ;3, and so on.
But basically, whether a heart or a happy face, the meaning usually comes out as love or happiness.
Ad more threes and exclamation marks to show that the emotion is more.
omgz! <333!! Thankyooo!
Less than three <3 and more than one >1