This T-Shirt says "POP'S" and possibly "finest beer" and I'm not sure what else. The t-shirt is light grey with black trim. Oddly enough, in subsequent reappearances, it appears to have lost its Logo. I’ve counted them as the same shirt, just because of the uniqueness of the black trim in both instances, but they are most likely two different shirts - so I've noted when the shirt appears with the POP'S logo and when it does not.
In the Pilot (1x01), Sam is wearing it to bed when Dean breaks into his apartment. He changes out of it before leaving with Dean to go to Jericho. (with logo)
In Dead In The Water (1x03), Sam wears this t-shirt while saying goodbye to Lucas and his mother at the end of the episode. (with logo)
In Skin (1x06), Sam wears it to say goodbye to Becky at the end of the episode.
In Home (1x09), Sam wears it while questioning John’s former business partner and then keeps it on through the main action of the episode, only changing for the ‘morning-after’ goodbye scene at the very end.
In Faith (1x12), Sam wears it underneath many layers, while researching reapers and thwarting Sue-Ann Le Grange.