Where are examples of infidelity found in The Great Gatsby, and how do the different characters demonstrate it?

The greatest infidelity found in the story is Tom's
because he is having an affair with Myrtle and isn't trying very hard to hide it.  In
fact, he practically flaunts it for Nick.  The proof comes from the
fact that he is keeping a secret "love nest" in the city for her and is buying her
expensive trinkets.  It is also implied, at one point, that Tom had even cheated on
Daisy shortly after they were married with one of the maids for the hotel (which is
discovered after the two are in a car accident in which she is
injured.)

Daisy, of course, is cheating on Tom with Gatsby.
 You never really see this in detail, but it is pretty well implied
because of the amount of time she is spending at his house and the efforts he goes
through to keep her visits on the "down low."  The story also describes how Daisy goes
around the table and kisses Gatsby while Tom is on the phone in the other room
(ironically with Myrtle, if I remember right.)  Unlike Tom, Gatsby tries very hard to
keep his affair with Daisy quiet by firing his potentially gossiping staff and ending
his weekend parties.  Daisy is a little less discrete, inviting Gatsby over for lunch
and such, but in some ways her cheating can be seen as almost retaliatory against
Tom.

You would have to add Myrtle cheating on her husband
George.  Though she doesn't seem very attached to him, and he seems pretty clueless to
her wandering for much of the affair, she is still playing house with Tom rather than
honoring her wedding vows.

In some ways, you could even
make a case that Daisy cheated on Gatsby.  They were involved in a relationship while he
was in World War I (and afterwards, while he was stuck in Europe,) and in the end she
rejected the relationship in order to get on with the life she felt she was entitled to.
 Though not a marital infidelity, she still pretty much ditched her
boyfriend.

Keeping with that definition, you could add Nick
to that pile.  Remember, Nick was cavorting around with Jordan while Nick has made clear
that he has a girlfriend back in the Midwest that he is expected to
marry.

So, as you can see, pretty much everybody is
unfaithful in one way or another in this story (other than maybe Jordan and
George.)