Dean knows how to wear plaid – the cozy flannel kind that’s your basic blue/grey. This shirt does up with snaps, not buttons, and the plaid on the breast pockets runs diagonal.
Dean wears it in his Djinn induced dream during What Is And What Should Never Be (2x20). We see his imagination wear it after the dinner where Sam and Jess reveal that they're engaged and Dean discovers that his dreamworld isn't perfect, up until Dean and Sam drive to the warehouse so that Dean can escape back to reality. (I'm still counting this as a new shirt for the 3rd season because that's the first time he wears it in reality, but it's good to note that he dreamt about it in S2, because that tells us that he already owned it at that point.)
Dean wears it in Fresh Blood (3x07) at the very end of the episode when he teaches Sam about the Impala.
In Dream A Little Dream Of Me (3x10), Dean wears it from the beginning of the episode until he has to put on a suit to go interview people who knew the dead professor.
In Time Is On My Side (3x15), Dean wears it when he is eating his burger in the motel room and Sam is going on and on about maggots. Bonus cap, because I love his face:
In Wishful Thinking (4x08), Dean is wearing it upon arrival in Concrete, WA. He keeps it on until he and Sam figure out that Wes was the first wisher, and then changes before they go to talk to him.
In Sex and Violence (4x14), Dean wears it for the final scene when he and Sam say goodbye to Bobby.
In Changing Channels (5x08), Dean wears it on the Japanese Game Show.
In Unforgiven (6x13), Dean wears this the day after Sam gets out of jail. He keeps it on until they kill the Arachne.
In The Born-Again Identity (7x17), Dean wears this shirt from when he goes to find “Emmanuel” until the end of the episode.
In As Time Goes By (8x12), Dean is wearing this shirt when Henry falls out of the closet. Dean continues to wear the shirt for the rest of the day, but takes it off before we see him sleeping in bed as Henry reads John’s journal.
In Rock and a Hard Place (9x08), Dean wears this shirt when he and Sam go to investigate the church that all the victims had in common. He wears it until Vesta is killed, (presumably only taking it off between scenes when he has sex with Suzie).
In Regarding Dean (12x11), Dean wears this shirt during his “drunken” night out, the hunt of the witch through the woods, and when he wakes up the next morning and gets waffles with Sam.